Colorado River#History

{{main|Course of the Colorado River}}

Image:Colorado_River_in_Rocky_Mountain_NP.jpg, Colorado]]

The Colorado begins at La Poudre Pass in the Never Summer Mountains in Rocky Mountain National Park, {{cvt|10184|ft|m}} above sea level.{{cite gnis |id=177510 |name=La Poudre Pass |entrydate=October 13, 1978 |access-date=March 12, 2012}} After a short run south, the river turns west below Grand Lake, the largest natural lake in the state.{{sfn|Lindberg|p=38|2009}} For the first {{convert|250|mi|km}} of its course, the Colorado carves its way through the mountainous Western Slope, a sparsely populated region defined by the portion of the state west of the Continental Divide. As it flows southwest, it gains strength from many small tributaries, as well as larger ones including the Blue, Eagle and Roaring Fork rivers. After passing through De Beque Canyon, the Colorado emerges from the Rockies into the Grand Valley, a major farming and ranching region where it meets one of its largest tributaries, the Gunnison River, at Grand Junction. Most of the upper river is a swift whitewater stream ranging from {{convert|200|to|500|ft|m|-1}} wide, the depth ranging from {{convert|6|to|30|ft|m|0}}, with a few notable exceptions, such as the Blackrocks reach where the river is nearly {{convert|100|ft|m}} deep.{{sfn|Benke|Cushing|p=490|2005}}

{{cite web |author1=Pitlick, John |author2=Cress, Robert |url=http://www.colorado.edu/geography/geomorph/usfws_reports/USFWS00.pdf |title=Longitudinal Trends in Channel Characteristics of the Colorado River and Implications for Food-Web Dynamics |publisher=University of Colorado |date=March 2000 |access-date=March 12, 2012 |archive-url=https://web.archive.org/web/20121204010148/http://www.colorado.edu/geography/geomorph/usfws_reports/USFWS00.pdf |archive-date=December 4, 2012 |url-status=dead}}

In a few areas, such as the marshy Kawuneeche Valley near the headwaters{{cite web |author=U.S. Geological Survey |title=United States Geological Survey Topographic Maps: Fall River Pass, Colorado quad |publisher=TopoQuest |url=http://www.topoquest.com/map-detail.php?usgs_cell_id=14931 |access-date=June 24, 2013 |archive-date=July 27, 2021 |archive-url=https://web.archive.org/web/20210727120303/https://www.topoquest.com/map-detail.php?usgs_cell_id=14931 |url-status=live }} and the Grand Valley, it exhibits braided characteristics.

From Grand Junction, the Colorado turns northwest before cutting southwest across the eponymous Colorado Plateau, a vast area of high desert centered at the Four Corners of the southwestern United States. Here, the climate becomes significantly drier than that in the Rocky Mountains, and the river becomes entrenched in progressively deeper gorges of bare rock, beginning with Ruby Canyon and then Westwater Canyon as it enters Utah, now once again heading southwest.{{cite web |author=U.S. Geological Survey |title=United States Geological Survey Topographic Maps: Westwater, Utah quad |publisher=TopoQuest |url=http://www.topoquest.com/map-detail.php?usgs_cell_id=48474 |access-date=July 14, 2013 |archive-date=July 27, 2021 |archive-url=https://web.archive.org/web/20210727233302/https://www.topoquest.com/map-detail.php?usgs_cell_id=48474 |url-status=live }} Farther downstream it receives the Dolores River and defines the southern border of Arches National Park, before passing Moab and flowing through "The Portal", where it exits the Moab Valley between a pair of {{convert|1000|ft|m|adj=on}} sandstone cliffs.{{cite web |url=http://www.gjem.energy.gov/moab/documents/eis/final_eis/Volume_I/Chapter3.pdf |title=Affected Environment |publisher=Moab, Utah, UMTRA Project |work=Remediation of the Moab Uranium Mill Tailings, Grand and San Juan Counties, Utah Final Environmental Impact Statement |page=28 |archive-url=https://web.archive.org/web/20111015041455/http://www.gjem.energy.gov/moab/documents/eis/final_eis/Volume_I/Chapter3.pdf |archive-date=October 15, 2011}}

Image:Grand Canyon view from Pima Point 2010.jpg]]

In Utah, the Colorado flows primarily through the "slickrock" country, which is characterized by its narrow canyons and unique "folds" created by the tilting of sedimentary rock layers along faults. This is one of the most inaccessible regions of the continental United States.{{cite web |url=http://geology.byu.edu/drupal/sites/default/files/volume-18-part-2-1971.pdf |title=Guidebook to the Colorado River Part 3: Moab to Hite, Utah through Canyonlands National Park |publisher=Brigham Young University |work=Brigham Young University Geology Studies Volume 18 – Part 2 |year=1971 |author1=Rigby, J. Keith |author2=Hamblin, W. Kenneth |author3=Matheny, Ray |author4=Welsh, Stanley L. |access-date=July 14, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20150402100700/http://geology.byu.edu/drupal/sites/default/files/volume-18-part-2-1971.pdf |archive-date=April 2, 2015}}{{sfn|Hopkins|2002|p=118}} Below the confluence with the Green River, its largest tributary, in Canyonlands National Park, the Colorado enters Cataract Canyon, named for its dangerous rapids,{{sfn|Van Cott|1990|p=70}} and then Glen Canyon, known for its arches and erosion-sculpted Navajo sandstone formations.{{sfn|Van Cott|1990|p=155}} Here, the San Juan River, carrying runoff from the southern slope of Colorado's San Juan Mountains, joins the Colorado from the east. The Colorado then enters northern Arizona, where since the 1960s Glen Canyon Dam near Page has flooded the Glen Canyon reach of the river, forming Lake Powell for hydroelectricity generation.{{cite web |url=http://www.nps.gov/cany/planyourvisit/rivers.htm |title=Rivers |publisher=U.S. National Park Service |work=Canyonlands National Park |date=June 17, 2011 |access-date=February 19, 2012 |archive-date=June 29, 2012 |archive-url=https://web.archive.org/web/20120629124634/http://www.nps.gov/cany/planyourvisit/rivers.htm |url-status=live }}{{cite web |url=http://www.usbr.gov/uc/rm/crsp/gc/lakepowell.html |title=Lake Powell |publisher=U.S. Bureau of Reclamation |work=Colorado River Storage Project |date=November 4, 2008 |access-date=February 19, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120921064853/http://www.usbr.gov/uc/rm/crsp/gc/lakepowell.html |archive-date=September 21, 2012}}

In Arizona, the river passes Lee's Ferry, an important crossing for early explorers and settlers and since the early 20th century the principal point where Colorado River flows are measured for apportionment to the seven U.S. and two Mexican states in the basin.{{cite encyclopedia |author=Rusho, W.L. |url=http://www.uen.org/utah_history_encyclopedia/l/LEES_FERRY.html |title=Lee's Ferry, Arizona |publisher=University of Utah |encyclopedia=Utah History Encyclopedia |year=1992 |access-date=February 19, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130109231047/http://www.uen.org/utah_history_encyclopedia/l/LEES_FERRY.html |archive-date=January 9, 2013}} Downstream, the river enters Marble Canyon, the beginning of the Grand Canyon, passing under the Navajo Bridges on a now southward course. Below the confluence with the Little Colorado River, the river swings west into Granite Gorge, the most dramatic portion of the Grand Canyon, where the river cuts up to {{convert|1|mi|km|spell=in}} into the Colorado Plateau, exposing some of the oldest visible rocks on Earth, dating as long ago as 2 billion years.{{cite web |url=http://www.nature.nps.gov/geology/parks/grca/ |title=Grand Canyon National Park, Arizona: Geology Fieldnotes |publisher=U.S. National Park Service |access-date=July 14, 2013 |archive-date=October 23, 2013 |archive-url=https://web.archive.org/web/20131023181117/http://www.nature.nps.gov/geology/parks/grca/ |url-status=dead}} The {{convert|277|mi|km}} of the river that flow through the Grand Canyon are largely encompassed by Grand Canyon National Park and are known for their difficult whitewater, separated by pools that reach up to {{convert|110|ft|m}} in depth.{{cite web |url=http://www.riversimulator.org/Resources/USGS/TheRapidsAndPoolsGrandCanyon.pdf |title=The Rapids and the Pools – Grand Canyon |author=Leopold, Luna B. |page=133 |publisher=U.S. Geological Survey |work=Geological Survey Professional Paper 669–D |access-date=November 2, 2013 |archive-date=November 4, 2013 |archive-url=https://web.archive.org/web/20131104061607/http://www.riversimulator.org/Resources/USGS/TheRapidsAndPoolsGrandCanyon.pdf |url-status=live }}

At the lower end of Grand Canyon, the Colorado widens into Lake Mead, the largest reservoir in the continental United States, formed by Hoover Dam on the border of Arizona and Nevada. Situated southeast of metropolitan Las Vegas, the dam is an integral component for management of the Colorado River, controlling floods and storing water for farms and cities in the lower Colorado River basin.{{cite web |url=http://www.usbr.gov/lc/hooverdam/faqs/lakefaqs.html |title=Hoover Dam Frequently Asked Questions and Answers: Lake Mead |publisher=U.S. Bureau of Reclamation |date=December 2008 |access-date=February 19, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120520123627/http://www.usbr.gov/lc/hooverdam/faqs/lakefaqs.html |archive-date=May 20, 2012}} Below the dam the river passes under the Mike O'Callaghan–Pat Tillman Memorial Bridge—which at nearly {{convert|900|ft|m}} above the water is the highest concrete arch bridge in the Western Hemisphere{{cite web |url=http://www.asce.org/OCEA-2012/ |title=Hoover Dam Bypass Is 2012 OCEA Winner; OPAL Lifetime Achievement Recipients Honored; Pankow, Michel Awards Presented |publisher=American Society of Civil Engineers |access-date=July 14, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20130327183408/http://www.asce.org/OCEA-2012/ |archive-date=March 27, 2013}}—and then turns due south towards Mexico, defining the Arizona–Nevada and Arizona–California borders.

Image:Yuma arizona map.jpg; Interstate 8 runs from left to right just below center.]]

After leaving the confines of the Black Canyon, the river emerges from the Colorado Plateau into the Lower Colorado River Valley (LCRV), a desert region dependent on irrigation agriculture and tourism and also home to several major Indian reservations.{{cite web |url=http://www.indiancountryextension.org/sites/indiancountryextension.org/files/publications/files/u6/CRIT%20Quick%20Facts%20Oct08.pdf |title=The Colorado River Indian Tribes Reservation (C.R.I.T.) Quick Facts |publisher=University of Arizona |work=Arizona Cooperative Extension |date=August 2010 |access-date=July 14, 2013 |archive-url=https://web.archive.org/web/20111021180950/http://indiancountryextension.org/sites/indiancountryextension.org/files/publications/files/u6/CRIT%20Quick%20Facts%20Oct08.pdf |archive-date=October 21, 2011 |url-status=dead}} The river widens here to a broad, moderately deep waterway averaging {{convert|500|to|1000|ft|m}} wide and reaching up to {{convert|0.25|mi|m|sigfig=1}} across, with depths ranging from {{convert|8|to|60|ft|m|sigfig=1}}.{{cite web |url=http://www.usbr.gov/lc/phoenix/reports/capdeis/Appendices/AppendixJ.pdf |title=Appendix J: Mainstem Impacts Analysis |publisher=U.S. Bureau of Reclamation |work=Draft Environmental Impact Statement: Allocation of Water Supply and Long-Term Contract Execution, Central Arizona Project |access-date=April 4, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130120035100/http://www.usbr.gov/lc/phoenix/reports/capdeis/Appendices/AppendixJ.pdf |archive-date=January 20, 2013}}{{cite web |author=U.S. Geological Survey |title=United States Geological Survey Topographic Maps: Blythe NE, California quad |publisher=TopoQuest |url=http://www.topoquest.com/map-detail.php?usgs_cell_id=4619 |access-date=July 14, 2013 |archive-date=July 27, 2021 |archive-url=https://web.archive.org/web/20210727120302/https://www.topoquest.com/map-detail.php?usgs_cell_id=4619 |url-status=live }} Before channelization of the Colorado in the 20th century, the lower river was subject to frequent course changes caused by seasonal flow variations. Joseph C. Ives, who surveyed the lower river in 1861, wrote that "the shifting of the channel, the banks, the islands, the bars is so continual and rapid that a detailed description, derived from the experiences of one trip, would be found incorrect, not only during the subsequent year, but perhaps in the course of a week, or even a day."{{sfn|Gupta|2007|p=204}}

The LCRV is one of the most densely populated areas along the river, and there are numerous towns including Bullhead City, Arizona, Needles, California, and Lake Havasu City, Arizona. Here, several large diversions draw from the river, providing water for both local uses and distant regions including the Salt River Valley of Arizona and metropolitan Southern California.{{cite web |url=http://fire.biol.wwu.edu/trent/alles/LowerColorado.pdf |title=The Lower Colorado River |editor=Alles, David L. |publisher=Western Washington University |work=Biology Department |date=October 28, 2011 |access-date=July 14, 2013 |archive-date=December 7, 2012 |archive-url=https://web.archive.org/web/20121207052523/http://fire.biol.wwu.edu/trent/alles/LowerColorado.pdf |url-status=live }} The last major U.S. diversion is at Imperial Dam, where over 90 percent of the river's flow is moved into the Gila Gravity Canal and Yuma Area Project, and the much bigger All-American Canal to irrigate California's Imperial Valley, the most productive winter agricultural region in the United States.{{cite news |url=https://www.npr.org/2011/02/10/133624969/fighting-for-water-in-the-arid-imperial-valley |title=Fighting For Water in Arid Imperial Valley |work=National Public Radio |date=February 10, 2011 |access-date=February 19, 2012 |archive-date=July 27, 2021 |archive-url=https://web.archive.org/web/20210727120257/https://www.npr.org/2011/02/10/133624969/fighting-for-water-in-the-arid-imperial-valley |url-status=live }}

Image:ColoradoRiverAtSanLuis.jpgBelow Imperial Dam, only a small portion of the Colorado River makes it beyond Yuma, Arizona, and the confluence with the intermittent Gila River—which carries runoff from western New Mexico and most of Arizona–before defining {{convert|24|mi|km}} of the Mexico–United States border. At Morelos Dam, the entire remaining flow of the Colorado is diverted to irrigate the Mexicali Valley, among Mexico's most fertile agricultural lands.{{cite web |url=http://wrrc.arizona.edu/sites/wrrc.arizona.edu/files/Arroyo-2012.pdf |title=Border Water Source of Conflict and Cooperation: What Makes Management of Border Water Resources a Challenge? |publisher=University of Arizona |work=Arroyo |year=2012 |access-date=July 14, 2013 |archive-date=November 13, 2014 |archive-url=https://web.archive.org/web/20141113235702/http://wrrc.arizona.edu/sites/wrrc.arizona.edu/files/Arroyo-2012.pdf |url-status=dead}} Below San Luis Río Colorado, the Colorado passes entirely into Mexico, defining the Baja California–Sonora border. Since 1960, the stretch of the Colorado between here and the Gulf of California has been dry or a trickle formed by irrigation return flows. The Hardy River provides most of the flow into the Colorado River Delta, a vast alluvial floodplain covering about {{convert|3000|mi2|km2}} of northwestern Mexico.{{cite web |author=Valdes-Casillas, Carlos |url=http://www.sci.sdsu.edu/salton/InfoDatabaseRioHardy.html |title=Information Database and Local Outreach Program for the Restoration of the Hardy River Wetlands, Lower Colorado River Delta, Baja California and Sonora, Mexico |publisher=San Diego State University |date=August 1998 |access-date=February 19, 2012 |display-authors=etal |archive-date=January 20, 2013 |archive-url=https://web.archive.org/web/20130120034144/http://www.sci.sdsu.edu/salton/InfoDatabaseRioHardy.html |url-status=live }} A large estuary is formed here before the Colorado empties into the Gulf about {{convert|75|mi|km|sigfig=2}} south of Yuma. Occasionally the International Boundary and Water Commission allows a springtime pulse flow to recharge the delta.{{cite news |url=https://www.nationalgeographic.com/news/2014/3/140322-colorado-river-delta-pulse-flow-morelos-dam-minute-319-water/ |archive-url=https://web.archive.org/web/20190901195745/https://www.nationalgeographic.com/news/2014/3/140322-colorado-river-delta-pulse-flow-morelos-dam-minute-319-water/ |url-status=dead |archive-date=September 1, 2019 |title=Historic "Pulse Flow" Brings Water to Parched Colorado River Delta |date=2014-03-24 |author=Brian Clark Howard |publisher=National Geographic |author-link=Brian Clark Howard}}

Before 20th-century development dewatered the lower Colorado, a major tidal bore was present in the delta and estuary; the first historical record was made by the Croatian missionary in Spanish service Father Ferdinand Konščak on July 18, 1746.{{cite conference |author=Chanson, Hubert |title=Environmental, Ecological and Cultural Impacts of Tidal Bores, Benaks, Bonos and Burros |url=http://espace.library.uq.edu.au/view/UQ:185349 |publisher=University of Queensland |conference=Proceedings of the International Workshop on Environmental Hydraulics IWEH09, Theoretical, Experimental and Computational Solutions |year=2009 |access-date=November 29, 2009 |archive-date=October 21, 2011 |archive-url=https://web.archive.org/web/20111021055750/http://espace.library.uq.edu.au/view/UQ:185349 |url-status=live }} During spring tide conditions, the tidal bore—locally called El Burro—formed in the estuary about Montague Island in Baja California and propagated upstream.{{sfn|Felger|Broyles|pp=519–520|2007}}

Image:San juan river entrenched meanders.jpg]]

Image:Mineral-Bottom.jpg

{{main|List of tributaries of the Colorado River}}

The Colorado is joined by over 25 significant tributaries, of which the Green River is the largest by both length and discharge. The Green River takes drainage from the Wind River Range of west-central Wyoming, from Utah's Uinta Mountains, and from the Rockies of northwestern Colorado.{{cite web |url=http://waterplan.state.wy.us/plan/green/execsumm.html |title=Executive Summary, Green River Basin Water Plan |publisher=Wyoming State Water Plan |date=February 2001 |access-date=February 25, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111125065800/http://waterplan.state.wy.us/plan/green/execsumm.html |archive-date=November 25, 2011}} The Gila River is the second longest and drains a greater area than the Green, but has a significantly lower flow because of a more arid climate and larger diversions for irrigation and cities.{{sfn|Benke|Cushing|p=514|2005}} Both the Gunnison and San Juan rivers, which derive most of their water from Rocky Mountains snowmelt, contribute more water than the Gila contributed naturally.{{sfn|Benke|Cushing|pp=532–533|2005}}

Image:CRBS_Executive_Summary_FINAL-01.png

{{Anchor|Colorado River Basin|Upper Basin|Lower Basin}}The Colorado River Basin consists of {{convert|246000|mi2|km2}}, making it the seventh largest drainage basin in North America.{{cite web |url=http://pubs.usgs.gov/of/1987/ofr87-242/ |author=Kammerer, J.C. |title=Largest Rivers in the United States |publisher=U.S. Geological Survey |date=May 1990 |access-date=July 2, 2010 |archive-date=March 21, 2007 |archive-url=https://web.archive.org/web/20070321022627/http://pubs.usgs.gov/of/1987/ofr87-242/ |url-status=live }} About {{convert|238600|mi2|km2}}, or 97 percent of the basin, is in the United States.{{cite web |title=Boundary Descriptions and Names of Regions, Subregions, Accounting Units and Cataloging Units |url=http://water.usgs.gov/GIS/huc_name.html |publisher=U.S. Geological Survey |access-date=February 19, 2012 |archive-date=April 9, 2020 |archive-url=https://web.archive.org/web/20200409085137/https://water.usgs.gov/GIS/huc_name.html |url-status=live }} The basin extends into western Colorado and New Mexico, southwestern Wyoming, eastern and southern Utah, southeastern Nevada and California, and most of Arizona. The areas drained within Baja California and Sonora in Mexico are very small and do not contribute significant runoff.{{sfn|Benke|Cushing|pp=483–486|2005}} Aside from the Colorado River Delta, the basin extends into Sonora at a few locations further east, including the headwaters of the Santa Cruz River and San Pedro River (both tributaries of the Gila River). For hydrological management purposes, the Colorado River Basin is divided into the Upper Basin (the drainage area above Lees Ferry), and the Lower Basin. The Upper Basin covers only 45 percent of the land area of the Colorado River Basin, but contributes 92 percent of the runoff.{{cite web |url=https://wwa.colorado.edu/sites/default/files/2021-06/ColoRiver_StateOfScience_WWA_2020_Chapter_2.pdf |title=Current Understanding of Colorado River Basin Climate and Hydrology |publisher=Western Water Assessment |date=April 2020 |access-date=May 24, 2022 |archive-date=January 20, 2022 |archive-url=https://web.archive.org/web/20220120152505/https://wwa.colorado.edu/sites/default/files/2021-06/ColoRiver_StateOfScience_WWA_2020_Chapter_2.pdf |url-status=live }}

The entire eastern boundary of the Colorado River Basin runs along the North American Continental Divide and is defined largely by the Rocky Mountains and the Rio Grande Basin. The Wind River Range in Wyoming marks the northern extent of the basin, and is separated from the Colorado Rockies by the endorheic Great Divide Basin in southwestern Wyoming. Streams that are nearby the east side of the divide drain into the Mississippi River and Rio Grande, while nearby areas north of the Wind River Range drain into the Columbia River. The western boundary of the Colorado River Basin is formed by various ranges and plateaus that border the Great Basin, including the Uinta Mountains and Wasatch Range. Major Great Basin watersheds bordering the Colorado River Basin are the Great Salt Lake and Sevier Lake watersheds. To the south, the Colorado River Basin borders several watersheds in Mexico draining into the Gulf of California, including the Sonoyta, Concepción, and Yaqui rivers.{{cite web |title=Watersheds (map) |publisher=Commission for Environmental Cooperation |year=2006 |url=http://www.cec.org/naatlas/img/NA-Watersheds.gif |archive-url=https://wayback.archive-it.org/all/20080414101144/http://www.cec.org/naatlas/img/NA-Watersheds.gif |archive-date=April 14, 2008 |url-status=dead}}{{cite web |url=http://www.lib.utexas.edu/maps/atlas_mexico/river_basins.jpg |title=River Basins of Mexico |publisher=University of Texas |year=1975 |access-date=February 19, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111107163257/http://www.lib.utexas.edu/maps/atlas_mexico/river_basins.jpg |archive-date=November 7, 2011}} Much of the basin is at high elevation; the mean elevation is {{convert|5500|ft|m}}.{{sfn|Benke|Cushing|pp=483–486|2005}}{{cite web |author=Fassnacht, S.R. |url=http://www.easternsnow.org/proceedings/2004/fassnacht2.pdf |title=Streamflow Predictability in the Upper Versus Lower Colorado River Sub-Basins |publisher=Eastern Snow Conference |year=2004 |access-date=February 19, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120623092924/http://www.easternsnow.org/proceedings/2004/fassnacht2.pdf |archive-date=June 23, 2012}} Lees Ferry, more than halfway along the Colorado River from its source, is {{convert|3150|ft|m}} above sea level.{{cite gnis |id=6959 |name=Lees Ferry (historical) |entrydate=February 8, 1980 |access-date=May 25, 2022}} The highest point in the Colorado River Basin is {{convert|14321|ft|m|adj=on}} Uncompahgre Peak in Colorado's San Juan Mountains, while some water from the river drains via irrigation run-off into California's Salton Sea, {{convert|236|ft|m}} below sea level.{{cite web |url=https://saltonsea.ca.gov/wp-content/uploads/2021/03/2021-Annual-Report_3-5-21.pdf |title=Annual Report on the Salton Sea Management Program |publisher=California Natural Resources Agency |date=March 2021 |access-date=May 26, 2022 |archive-date=April 1, 2022 |archive-url=https://web.archive.org/web/20220401033458/https://saltonsea.ca.gov/wp-content/uploads/2021/03/2021-Annual-Report_3-5-21.pdf |url-status=live }}

Image:Grand_Canyon_National_Park_Colorado_River_Black_Bridge_0618_(6697170671).jpg crosses the Colorado River in the Grand Canyon at Phantom Ranch, Arizona.]]

About 72 percent of the Colorado River Basin is classified as arid, with the Sonoran and Mojave deserts covering the southern portion and the Colorado Plateau encompassing much of the central portion.{{cite journal |author1=Singh, Ramesh K. |author2=Senay, Gabriel B. |author3=Velpuri, Naga Manohar |author4=Kagone, Stephanie |title=Actual Evapotranspiration (Water Use) Assessment of the Colorado River Basin at the Landsat Resolution Using the Operational Simplified Surface Energy Balance Model |journal=Remote Sensing |volume=6 |issue=1 |pages=233–256 |date=January 2014 |doi=10.3390/rs6010233 |url=https://www.researchgate.net/publication/259481061 |doi-access=free |access-date=May 26, 2022 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301090243/https://www.researchgate.net/publication/259481061_Actual_Evapotranspiration_Water_Use_Assessment_of_the_Colorado_River_Basin_at_the_Landsat_Resolution_Using_the_Operational_Simplified_Surface_Energy_Balance_Model |url-status=live }} The Colorado Plateau is home to most of the major canyon systems formed by the Colorado River and its tributaries, particularly those of the Green and San Juan rivers. About 23 percent of the basin is forest, with the largest area in the Rocky Mountains; other significant forested areas include the Kaibab, Aquarius, and Markagunt plateaus in southern Utah and northern Arizona, and the Mogollon Rim in central Arizona. These high plateaus and escarpments, often exceeding {{convert|9000|ft|m}} in elevation, form the northern and southern edges of the Colorado Plateau geological province.{{sfn|Benke|Cushing|pp=483–489|2005}}{{cite web |url=http://www.fs.fed.us/rm/pubs/rmrs_p013/rmrs_p013_117_128.pdf |title=Contributions of Watershed Management Research to Ecosystem-Based Management in the Colorado River Basin |publisher=U.S. Forest Service |year=2000 |work=USDA Forest Service Proceedings RMRS–P–13 |access-date=June 24, 2013 |author1=Baker Jr., Malchus B. |author2=Ffolliott, Peter F. |archive-date=October 23, 2020 |archive-url=https://web.archive.org/web/20201023000742/https://www.fs.fed.us/rm/pubs/rmrs_p013/rmrs_p013_117_128.pdf |url-status=live }} Developed land use in the basin is mostly irrigated agriculture, chiefly in the Grand Valley, the Lower Colorado River Valley, and the Salt River Valley, but the total area of crop and pasture land is only 2–3 percent of the entire basin. Urban areas cover less than 1 percent.

Climate in the Colorado River Basin ranges from subtropical hot desert at southern, lower elevations to alpine in the Rocky Mountains.{{cite book |url=https://nap.nationalacademies.org/read/11857/chapter/5 |title=Climate and Hydrology of the Colorado River Basin Region |publisher=National Academies Press |year=2007 |doi=10.17226/11857 |isbn=978-0-309-10524-8 |access-date=May 23, 2022 |archive-date=May 26, 2022 |archive-url=https://web.archive.org/web/20220526151202/https://nap.nationalacademies.org/read/11857/chapter/5 |url-status=live }} Mean monthly high temperatures are {{convert|25.3|C|F}} in the Upper Basin and {{convert|33.4|C|F}} in the Lower Basin, and lows average {{convert|-3.6|and|8.9|C|F}}, respectively. Annual precipitation averages {{convert|164|mm|in|order=flip}}, ranging from over {{convert|60|in|mm}} in some areas of the Rockies to less than {{convert|4|in|mm}} in dry desert valleys. The Upper Basin generally receives snow and rain during the winter and early spring, while precipitation in the Lower Basin falls mainly during intense but infrequent summer thunderstorms brought on by the North American Monsoon.{{cite book |chapter-url=http://www.nap.edu/openbook.php?record_id=11857&page=74 |chapter=Features and Dynamics of Colorado River Basin Climate |publisher=National Academies Press |title=Colorado River Basin Water Management: Evaluating and Adjusting to Hydroclimatic Variability |year=2007 |access-date=February 19, 2012 |doi=10.17226/11857 |isbn=978-0-309-10524-8 |archive-date=January 20, 2013 |archive-url=https://web.archive.org/web/20130120034145/http://www.nap.edu/openbook.php?record_id=11857&page=76 |url-status=live }} Precipitation is influenced by the El Niño-Southern Oscillation (ENSO) with El Niño being associated with wetter conditions and La Niña with drier conditions. The effect of ENSO is significantly more pronounced in the Lower Basin, where it has a strong impact on monsoonal rainfall. Runoff patterns across the Colorado River Basin reflect this; most of the perennial tributaries originate in the Upper Basin, while tributaries in the Lower Basin are either ephemeral (such as the Little Colorado River) or highly seasonal (such as the Gila and Salt rivers).

As of 2010, approximately 13 million people lived within the Colorado River basin,{{refn|American population (9.7 million) calculated from statistics from the U.S. Census Bureau{{cite web |url=http://quickfacts.census.gov/qfd/index.html |title=State and County QuickFacts |publisher=U.S. Census Bureau |access-date=March 4, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120304192030/http://quickfacts.census.gov/qfd/index.html |archive-date=March 4, 2012}} and the State of Colorado.{{cite web |url=http://www.colorado.gov/cs/Satellite?blobcol=urldata&blobheader=application%2Fpdf&blobheadername1=Content-Disposition&blobheadername2=MDT-Type&blobheadervalue1=inline%3B+filename%3D555%2F20%2FDB2007-Pop.pdf&blobheadervalue2=abinary%3B+charset%3DUTF-8&blobkey=id&blobtable=MungoBlobs&blobwhere=1191378405012&ssbinary=true |title=Population and Demographics |publisher=State of Colorado |work=Colorado Data Book |date=September 2007 |format=PDF |archive-url=https://web.archive.org/web/20130120034135/http://www.colorado.gov/cs/Satellite?blobcol=urldata&blobheader=application%2Fpdf&blobheadername1=Content-Disposition&blobheadername2=MDT-Type&blobheadervalue1=inline%3B%2Bfilename%3D555%2F20%2FDB2007-Pop.pdf&blobheadervalue2=abinary%3B%2Bcharset%3DUTF-8&blobkey=id&blobtable=MungoBlobs&blobwhere=1191378405012&ssbinary=true |archive-date=January 20, 2013 |url-status=dead}} The population in Mexico is about 3 million.{{cite web |url=https://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/programs/alphabetical/?&cid=stelprdb1044198 |title=Colorado River Basin Salinity Control Program |publisher=U.S. Department of Agriculture |work=Natural Resources Conservation Service |date=July 12, 2010 |access-date=March 4, 2012 |archive-date=June 10, 2021 |archive-url=https://web.archive.org/web/20210610152204/https://www.nrcs.usda.gov/wps/portal/nrcs/detail/national/programs/alphabetical/?&cid=stelprdb1044198 |url-status=live }}|group=n}} while about 40 million people live in areas supplied by Colorado River water. Colorado River basin states are among the fastest-growing in the US; the population of Nevada alone increased by about 66 percent between 1990 and 2000 as Arizona grew by some 40 percent.{{cite web |url=http://dels.nas.edu/resources/static-assets/materials-based-on-reports/reports-in-brief/colorado_river_management_final.pdf |title=Colorado River Basin Water Management: Evaluating and Adjusting to Hydroclimatic Variability (Report in Brief) |publisher=National Academies |work=Division on Earth & Life Studies |date=February 2007 |access-date=February 19, 2012 |archive-date=September 15, 2012 |archive-url=https://web.archive.org/web/20120915073458/http://dels.nas.edu/resources/static-assets/materials-based-on-reports/reports-in-brief/colorado_river_management_final.pdf |url-status=dead}} Phoenix, Arizona, Las Vegas, Nevada and Mexicali, Baja California are the largest cities by population within the Colorado River Basin. Other significant cities include Tucson, Arizona, St. George, Utah and Flagstaff, Arizona. Due to the rugged and inhospitable topography through which the river flows, there are only a few major towns along the Colorado River itself, including Grand Junction, Colorado and Yuma, Arizona.{{sfn|Benke|Cushing|pp=483–486|2005}}

Image:Coloradoflowgraph.png

The unimpaired annual runoff{{#tag:ref|Unimpaired runoff figures published by the Bureau of Reclamation are an estimate of unregulated river flow without any upstream water consumption.|group=n}} of the Colorado River at Lees Ferry, Arizona is estimated at {{convert|14.7|e6acre.ft|km3}}, or an annualized discharge of {{convert|20300|cuft/s|m3/s|abbr=on}}, for the 1906–2023 period.{{cite web|url=https://www.usbr.gov/lc/region/g4000/NaturalFlow/provisional.html|title=Colorado River Basin Natural Flow and Salt Data|publisher=U.S. Bureau of Reclamation|date=April 22, 2024|accessdate=August 11, 2024}} River flows at the Lees Ferry stream gauge, about halfway along the length of the Colorado and {{convert|16|mi|km}} below Glen Canyon Dam, are used to determine water allocations in the Colorado River basin.{{cite web |url=http://nwis.waterdata.usgs.gov/nwis/monthly/?referred_module=sw&site_no=09380000&por_09380000_1=19133,00060,1,1921-10,2011-09&format=html_table&date_format=YYYY-MM-DD&rdb_compression=file&submitted_form=parameter_selection_list |title=USGS Gage #09380000 on the Colorado River at Lee's Ferry, AZ |publisher=U.S. Geological Survey |work=National Water Information System |date=1921–2010 |access-date=April 21, 2012 |archive-date=February 22, 2016 |archive-url=https://web.archive.org/web/20160222154525/http://nwis.waterdata.usgs.gov/nwis/monthly/?referred_module=sw&site_no=09380000&por_09380000_1=19133,00060,1,1921-10,2011-09&format=html_table&date_format=YYYY-MM-DD&rdb_compression=file&submitted_form=parameter_selection_list |url-status=live}} Flows originating above Lees Ferry represent the majority of Colorado River runoff. Tributaries between there and Imperial Dam near the US–Mexico border contribute a further {{convert|1.1|e6acre.ft|km3}}, and the Gila–Salt River system historically accounted for another {{convert|1.7|e6acre.ft|km3}}.{{cite web|url=https://www.ansac.az.gov/UserFiles/PDF/02162016/C043-SLD-AG%20Supplement%206/369%20-%20Predevelopment%20Hydrology%20of%20the%20Gila%20River%20Indian%20Res%20South%20Central%20AZ.pdf|title=Predevelopment hydrology of the Gila River Indian Reservation, South-Central Arizona|author1=Thomsen, B.W. |author2=Eychaner, J.H.|year=1991|publisher=Arizona Navigable Stream Adjudication Commission|accessdate=August 11, 2024}}{{cite web|url=https://www.treeflow.info/sites/default/files/phillipsetal2009srpapp.pdf|title=Water resources planning and management at the Salt River Project, Arizona, USA|author=Phillips, Daniel H.; Reinink, Yvonne; Skarupa, Timothy E.; Ester III, Charles E.; and Skindlov, Jon A.|date=April 29, 2009|publisher=TreeFlow|accessdate=August 11, 2024}} While much of this water was lost to evaporation in the deserts of the Lower Basin, anywhere from {{convert|10|to|20|e6acre.ft|km3}} per year reached the Colorado River Delta prior to the early 20th century.{{cite web|url=https://water.utah.gov/wp-content/uploads/2019/01/TheColoradoRiverart.pdf|title=The Colorado River: Utah's Perspective|author=Anderson, D. Larry|year=2002|publisher=Utah Division of Water Resources|accessdate=August 11, 2024}}

Up to 90 percent of the river's flow originates as snowmelt from the Rocky Mountains and other smaller mountain ranges of the Colorado Plateau. Natural seasonal flows at its mouth varied widely, often exceeding {{convert|100000|cuft/s|m3/s|abbr=on}} in the spring melt and falling as low as {{convert|2500|cuft/s|m3/s|abbr=on}} in winter.{{sfn|Wiltshire|Gilbert|Rogers|2010|pp=101–102}} Snowmelt typically begins in April, peaks in May or June, and can extend into August in wet years.{{cite web |author1=Wahl, Kenneth L. |author2=Collins, Dannie L. |author3=Aldridge, Byron N. |url=http://www.westernsnowconference.org/proceedings/pdf_Proceedings/1987%20WEB/WahlUpperColoradoRiverBasinForecastsOfRunoff.pdf |title=Runoff in the Upper Colorado River Basin Water Years, 1983–85, and Comparisons with Forecasts of Runoff to Lake Powell |publisher=Western Snow Conference |archive-url=https://web.archive.org/web/20120714190407/http://www.westernsnowconference.org/proceedings/pdf_Proceedings/1987%20WEB/WahlUpperColoradoRiverBasinForecastsOfRunoff.pdf |archive-date=July 14, 2012}} Monsoon rainstorms often occur in the Lower Basin from August through October, contributing much of the flow in tributaries below Lees Ferry and often causing flash flooding.{{cite news |last=Preston |first=Ben |url=https://psmag.com/environment/the-risky-business-of-slicing-the-pie-11068 |title=The Risky Business of Slicing the Pie |work=Miller–McCune |date=March 22, 2010 |access-date=June 27, 2013 |archive-date=July 27, 2021 |archive-url=https://web.archive.org/web/20210727120256/https://psmag.com/environment/the-risky-business-of-slicing-the-pie-11068 |url-status=live }} Since the initial closure of Hoover Dam in 1934, lower Colorado River flows have been greatly moderated, with monthly means below the dam ranging from {{convert|16400|cuft/s|m3/s|abbr=on}} in May to {{convert|11200|cuft/s|m3/s|abbr=on}} in October.{{cite web|url=https://nwis.waterdata.usgs.gov/nwis/monthly/?referred_module=sw&site_no=09421500&por_09421500_102357=172091,00060,102357,1934-04,2021-09&format=html_table&date_format=YYYY-MM-DD&rdb_compression=file&submitted_form=parameter_selection_list|title=USGS Gage #09421500 on the Colorado River below Hoover Dam |publisher=U.S. Geological Survey |work=National Water Information System |date=1934–2021 |access-date=August 11, 2024}} Upstream of the massive Hoover and Glen Canyon Dams, inflows to Lake Powell still experience distinct spring highs and winter lows, though overall volumes have been reduced due to Upper Basin water diversions.Schmidt, J. C., Yackulic, C. B., & Kuhn, E. (2023). The Colorado River water crisis: Its origin and the future. WIREs Water, 10(6), e1672. https://doi.org/10.1002/wat2.1672 Flow regimes in a few Upper Basin tributaries, such as the Yampa River, remain almost completely unaltered.{{cite web|url=https://cpw.state.co.us/Documents/Research/Aquatic/pdf/CDOWTechReport44.pdf|title=Channel-forming discharge on the Dolores River and Yampa River, Colorado|author=Richard, Gigi A. and Anderson, Richard M.|publisher=Colorado Parks and Wildlife|year=2007|accessdate=August 11, 2024}}

By the 1950s, the Colorado delta was regularly drying up in fall and winter, though spring high flows often still made it to the ocean. After the initial closure of Glen Canyon Dam in 1963, these seasonal flows were almost completely eliminated, with the exception of a few very wet years, such as 1983–1987.{{cite journal |url=https://portal.azoah.com/oedf/documents/08A-AWS001-DWR/Supplemental_Beverly_et_al/20040000%20Christensen%20et%20al%20Effects%20of%20Climate%20Change%20on%20Hydrology%20and%20Water%20Resources_Colorado%20River%20B.pdf |title=The Effects of Climate Change on the Hydrology and Water Resources of the Colorado River Basin |journal=Climatic Change |volume=62 |number=337–363 |author1=Christensen, Niklas S. |author2=Wood, Andrew W. |author3=Voisin, Nathalie |author4-link=Dennis P. Lettenmaier |author4=Lettenmaier, Dennis P. |author5=Palmer, Richard N. |year=2004 |page=345 |doi=10.1023/b:clim.0000013684.13621.1f |bibcode=2004ClCh...62..337C |s2cid=53533021 |access-date=July 13, 2013 |archive-date=May 1, 2014 |archive-url=https://web.archive.org/web/20140501135832/https://portal.azoah.com/oedf/documents/08A-AWS001-DWR/Supplemental_Beverly_et_al/20040000%20Christensen%20et%20al%20Effects%20of%20Climate%20Change%20on%20Hydrology%20and%20Water%20Resources_Colorado%20River%20B.pdf |url-status=dead}}{{cite journal |url=http://www.pacinst.org/wp-content/uploads/2013/02/missing_water_article_web3.pdf |title=A Preliminary Water Balance for the Colorado River Delta, 1992–1998 |journal=Journal of Arid Environments |year=2001 |volume=49 |issue=1 |pages=35–48 |doi=10.1006/jare.2001.0834 |access-date=June 27, 2013 |author1=Cohen, Michael J. |author2=Henges-Jeck, Christine |author3=Castillo-Moreno, Gerardo |bibcode=2001JArEn..49...35C |archive-date=October 10, 2015 |archive-url=https://web.archive.org/web/20151010085822/http://pacinst.org/wp-content/uploads/2013/02/missing_water_article_web3.pdf |url-status=live }}{{cite web |url=http://pubs.usgs.gov/fs/2004/3062/ |title=Climatic Fluctuations, Drought, and Flow in the Colorado River Basin |publisher=U.S. Geological Survey |work=USGS Fact Sheet 2004-3062 |date=August 2004 |access-date=April 17, 2012 |archive-date=October 19, 2012 |archive-url=https://web.archive.org/web/20121019144633/http://pubs.usgs.gov/fs/2004/3062/ |url-status=live }}{{Cite web |date=2018-02-19 |title=For A Few Weeks The Colorado River Reached The Ocean. Will It Happen Again? |url=https://www.kpbs.org/news/midday-edition/2018/02/19/few-weeks-colorado-river-reached-ocean-will-it-hap |access-date=2022-02-27 |website=KPBS Public Media |archive-date=March 24, 2022 |archive-url=https://web.archive.org/web/20220324032421/https://www.kpbs.org/news/midday-edition/2018/02/19/few-weeks-colorado-river-reached-ocean-will-it-hap |url-status=live }} In 1984, {{convert|16.5|e6acre.ft|km3}} of excess runoff reached the ocean.{{cite web |url=http://nwis.waterdata.usgs.gov/nwis/monthly/?referred_module=sw&site_no=09522200&por_09522200_1=19984,00060,1,1960-10,1986-09&format=html_table&date_format=YYYY-MM-DD&rdb_compression=file&submitted_form=parameter_selection_list |title=USGS Gage #09522200 on the Colorado River at NIB, Above Morelos Dam, AZ – Annual Statistics |publisher=U.S. Geological Survey |work=National Water Information System |date=1951–1992 |access-date=April 27, 2012 |archive-date=September 19, 2015 |archive-url=https://web.archive.org/web/20150919054254/http://nwis.waterdata.usgs.gov/nwis/monthly/?referred_module=sw&site_no=09522200&por_09522200_1=19984,00060,1,1960-10,1986-09&format=html_table&date_format=YYYY-MM-DD&rdb_compression=file&submitted_form=parameter_selection_list |url-status=live }} Flows have further declined due to evaporation from reservoirs and, particularly after 2000, reduced snowpack with warming winter temperatures in the Rockies.{{cite news |author=Zimmerman, Janet |url=http://www.pe.com/local-news/topics/topics-environment-headlines/20101006-region-colorado-river-drought-threatens-power-production.ece |title=Colorado River Drought Threatens Power Production |work=The Press-Enterprise |location=Riverside, California |date=October 6, 2010 |access-date=February 18, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130120034136/http://www.pe.com/local-news/topics/topics-environment-headlines/20101006-region-colorado-river-drought-threatens-power-production.ece |archive-date=January 20, 2013}}{{cite news |author=Stark, Mike |agency=Associated Press |url=http://usatoday30.usatoday.com/weather/drought/2009-04-21-colorado-river-shortages_N.htm |title=Study: Shortages Likely on Colorado River by 2050 |newspaper=USA Today |date=April 21, 2009 |access-date=June 27, 2013 |archive-date=July 18, 2013 |archive-url=https://web.archive.org/web/20130718113458/http://usatoday30.usatoday.com/weather/drought/2009-04-21-colorado-river-shortages_N.htm |url-status=live }}{{cite web |author1=Hoerling, Martin |author2=Eischeid, Jon |url=http://wwa.colorado.edu/climate_change/docs/hoerling%20past%20peak%20water%20in%20press.pdf |title=Past Peak Water in the Southwest |publisher=Western Water Assessment |archive-url=https://web.archive.org/web/20100609223341/http://wwa.colorado.edu/climate_change/docs/hoerling%20past%20peak%20water%20in%20press.pdf |archive-date=June 9, 2010}} Water diversions have effectively removed some tributaries entirely, including the Gila River, once the Colorado's largest tributary below Lees Ferry.{{cite news |author=McKinnon, Shaun |url=http://www.azcentral.com/specials/special06/articles/0809rivers0809NEW.html |title=Mines, Farms Put Gila River on Life Support: Despite Lack of Regulation, Some Trying to Restore River to Its Natural Beauty and Wonder |work=Arizona Republic |date=August 9, 2006 |access-date=February 19, 2012 |archive-date=January 17, 2013 |archive-url=https://archive.today/20130117125841/http://www.azcentral.com/specials/special06/articles/0809rivers0809NEW.html |url-status=dead}} Most Colorado River water is diverted in the United States, with only about {{convert|2.8|e6acre.ft|km3}} per year reaching Mexico since 1950. At Morelos Dam, the remaining flow is diverted to irrigate the Mexicali Valley, leaving the Colorado River Delta almost entirely dry except for small amounts of agricultural wastewater.{{cite web |url=http://www.crwua.org/colorado-river/uses/reclamation |title=Reclamation |publisher=Colorado River Water Users Association |access-date=November 2, 2013 |archive-url=https://web.archive.org/web/20131104165659/http://www.crwua.org/colorado-river/uses/reclamation |archive-date=November 4, 2013 |url-status=dead}}

As recently as the Cretaceous period about 100 million years ago, much of western North America was still part of the Pacific Ocean. Tectonic forces from the collision of the Farallon Plate with the North American Plate pushed up the Rocky Mountains between 50 and 75 million years ago in a mountain-building episode known as the Laramide orogeny.{{sfn|Prisciantelli|2002|p=56}} The Colorado River first formed as a west-flowing stream draining the southwestern portion of the range, and the uplift also diverted the Green River, once a tributary of the Mississippi River, west towards the Colorado. About 30 to 20 million years ago, volcanic activity related to the orogeny led to the Mid-Tertiary ignimbrite flare-up, which created smaller formations such as the Chiricahua Mountains in Arizona and deposited massive amounts of volcanic ash and debris over the watershed.{{cite web |author=Cannon, Eric |url=http://www.colorado.edu/GeolSci/Resources/WUSTectonics/CzIgnimbrite/ignimbrite_intro.html |title=The Mid-Tertiary Ignimbrite Flare-Up |publisher=University of Colorado, Boulder |date=April 28, 2002 |access-date=February 19, 2012 |archive-url=https://web.archive.org/web/20120716192901/http://www.colorado.edu/GeolSci/Resources/WUSTectonics/CzIgnimbrite/ignimbrite_intro.html |archive-date=July 16, 2012 |url-status=dead}} The Colorado Plateau first began to rise during the Eocene, between about 55 and 34 million years ago, but did not attain its present height until about 5 million years ago, about when the Colorado River established its present course into the Gulf of California.{{cite web |author=Foos, Annabelle |url=http://www.nature.nps.gov/geology/education/foos/plateau.pdf |title=Geology of the Colorado Plateau |publisher=U.S. National Park Service |access-date=February 19, 2012 |archive-date=September 8, 2015 |archive-url=https://web.archive.org/web/20150908124057/http://nature.nps.gov/geology/education/foos/plateau.pdf |url-status=dead}}

The time scale and sequence over which the river's present course and the Grand Canyon were formed is uncertain. Before the Gulf of California was formed around 12 to 5 million years ago by faulting processes along the boundary of the North American and Pacific plates,{{cite web |author=Martin-Barajas, Arturo |url=http://www.gps.caltech.edu/~jstock/Bajares.html |title=The Tectonic Evolution of the Gulf of California |publisher=California Institute of Technology |work=Division of Geological and Planetary Sciences |date=July 2009 |access-date=February 19, 2012 |display-authors=etal |archive-date=January 20, 2013 |archive-url=https://web.archive.org/web/20130120034145/http://www.gps.caltech.edu/~jstock/Bajares.html |url-status=live }} the Colorado flowed west to an outlet on the Pacific Ocean—possibly Monterey Bay on the Central California coast, and may have played a role in the formation of the Monterey submarine canyon. Crustal extension in the Basin and Range Province began about 20 million years ago and the modern Sierra Nevada began forming about 10 million years ago, eventually diverting the Colorado southwards towards the Gulf.{{sfn|Young|Spamer|p=193|2001}} As the Colorado Plateau continued to rise between 5 and 2.5 million years ago, the river maintained its ancestral course (as an antecedent stream) and began to cut the Grand Canyon. Antecedence played a major part in shaping other peculiar geographic features in the watershed, including the Dolores River's bisection of Paradox Valley in Colorado and the Green River's cut through the Uinta Mountains in Utah.{{cite journal |author=Davis, Jim |url=http://geology.utah.gov/surveynotes/gladasked/gladriver.htm |title=Why Does a River Run Through It? |publisher=Utah Geological Survey |journal=Survey Notes |volume=41 |date=January 2009 |access-date=February 19, 2012 |issue=1 |archive-date=November 8, 2010 |archive-url=https://web.archive.org/web/20101108171715/http://geology.utah.gov/surveynotes/gladasked/gladriver.htm |url-status=live }}

Image:VulcanUinkart.jpg are seen here descending into the Grand Canyon, where they dammed the Colorado over 10 times in the past 2 million years.]]

Sediments carried from the plateau by the Colorado River created a vast delta made of more than {{convert|10000|mi3|km3}} of material that walled off the northernmost part of the gulf in approximately 1 million years. Cut off from the ocean, the portion of the gulf north of the delta eventually evaporated and formed the Salton Sink, which reached about {{convert|260|ft|m}} below sea level.{{sfn|Prisciantelli|2002|p=62}}{{cite web |url=http://fire.biol.wwu.edu/trent/alles/GeologySaltonTrough.pdf |title=Geology of the Salton Trough |editor=Alles, David L. |publisher=Western Washington University |date=October 28, 2011 |access-date=February 29, 2012 |archive-date=March 2, 2012 |archive-url=https://web.archive.org/web/20120302204509/http://fire.biol.wwu.edu/trent/alles/GeologySaltonTrough.pdf |url-status=live }} Since then the river has changed course into the Salton Sink at least three times, transforming it into Lake Cahuilla, which at maximum size flooded up the valley to present-day Indio, California. The lake took about 50 years to evaporate after the Colorado resumed flowing to the Gulf. The present-day Salton Sea can be considered the most recent incarnation of Lake Cahuilla, though on a much smaller scale.{{cite web |author1=Buckles, Joseph E. |author2=Krantz, Timothy |url=http://proceedings.esri.com/library/userconf/proc00/professional/papers/PAP751/p751.htm |title=Reconstruction of Prehistoric Shorelines for Cultural Restraints using GIS |publisher=ESRI |work=Salton Sea Database Program |access-date=February 19, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111127002625/http://proceedings.esri.com/library/userconf/proc00/professional/papers/PAP751/p751.htm |archive-date=November 27, 2011}}

Between 1.8 million and 10,000 years ago, massive flows of basalt from the Uinkaret volcanic field in northern Arizona dammed the Colorado River within the Grand Canyon. At least 13 lava dams were formed, the largest of which was more than {{convert|2300|ft|m}} high, backing the river up for nearly {{convert|500|mi|km}} to present-day Moab, Utah.{{cite web |author1=Fenton, Cassie |author2=Webb, Bob |url=http://www.gcrg.org/bqr/16-4/dams.html |title=A Fresh Look at Western Grand Canyon Lava Dams: Introduction |publisher=Grand Canyon River Guides |access-date=February 19, 2012 |archive-url=https://web.archive.org/web/20120623033347/http://www.gcrg.org/bqr/16-4/dams.html |archive-date=June 23, 2012 |url-status=dead}} The lack of associated sediment deposits along this stretch of the Colorado River, which would have accumulated in the impounded lakes over time, suggests that most of these dams did not survive for more than a few decades before collapsing or being washed away. Failure of the lava dams caused by erosion, leaks and cavitation caused catastrophic floods, which may have been some of the largest ever to occur in North America, rivaling the late-Pleistocene Missoula Floods of the northwestern United States.{{cite web |author1=Rugg, Scott H. |author2=Austin, Steven A. |url=http://static.icr.org/i/pdf/technical/Lava-Dams-of-the-Western-Grand-Canyon-Arizona.pdf |title=Evidence for Rapid Formation and Failure of Pleistocene "Lava Dams" of the Western Grand Canyon, Arizona |publisher=Institute for Creation Research |date=August 1998 |access-date=February 19, 2012 |archive-url=https://web.archive.org/web/20120121063054/http://static.icr.org/i/pdf/technical/Lava-Dams-of-the-Western-Grand-Canyon-Arizona.pdf |archive-date=January 21, 2012 |url-status=dead}} Mapping of flood deposits indicate that crests as high as {{convert|700|ft|m}} passed through the Grand Canyon,{{cite journal |url=http://www.ees.rochester.edu/RareGasLab/Fenton_etal_JGeology.pdf |title=Geochemical Discrimination of Five Pleistocene Lava-Dam Outburst-Flood Deposits, Western Grand Canyon, Arizona |journal=Journal of Geology |year=2004 |volume=112 |issue=1 |pages=91–110 |access-date=February 19, 2012 |author=Fenton, Cassandra R. |doi=10.1086/379694 |last2=Poreda |first2=Robert J. |last3=Nash |first3=Barbara P. |last4=Webb |first4=Robert H. |last5=Cerling |first5=Thure E. |display-authors=etal |bibcode=2004JG....112...91F |s2cid=53339467 |archive-date=January 20, 2013 |archive-url=https://web.archive.org/web/20130120034145/http://www.ees.rochester.edu/RareGasLab/Fenton_etal_JGeology.pdf |url-status=live }} reaching peak discharges as great as {{convert|17|e6cuft/s|m3/s}}.{{cite journal |author1=Fenton, Cassandra R. |author2=Webb, Robert H |author3=Cerling, Thure E. |url=http://wwwpaztcn.wr.usgs.gov/webb_pdf/Fenton-ea-2005-QR.pdf |title=Peak Discharge of a Pleistocene Lava-Dam Outburst Flood in Grand Canyon, Arizona, USA |journal=Quaternary Research |volume=65 |issue=2 |pages=324 |publisher=U.S. Geological Survey |date=May 6, 2005 |access-date=February 19, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111015115627/http://wwwpaztcn.wr.usgs.gov/webb_pdf/Fenton-ea-2005-QR.pdf |archive-date=October 15, 2011 |bibcode=2006QuRes..65..324F |doi=10.1016/j.yqres.2005.09.006 |s2cid=128533268}}

Image:Cliff_Palace-Colorado-Mesa_Verde_NP.jpg]]

Small numbers of Paleo-Indians of the Clovis and Folsom cultures inhabited the Colorado Plateau as early as 10,000 BCE, with populations beginning to increase in the Desert Archaic period (6000 BCE–0 CE).{{cite web |url=http://cpluhna.nau.edu/People/people.htm |title=People of the Colorado Plateau |publisher=Northern Arizona University |work=Land Use History of North America |access-date=April 9, 2012 |archive-date=June 29, 2015 |archive-url=https://web.archive.org/web/20150629182507/http://www.cpluhna.nau.edu/People/people.htm}} While most early inhabitants were hunter-gatherers, evidence of agriculture, masonry dwellings and petroglyphs begins with the Fremont culture period (0–1300 CE). The Ancient Puebloan culture, also known as Anasazi or Hisatsinom, were descended from the Desert Archaic culture and became established in the Four Corners region around 1000 CE.{{cite web |url=http://www.blm.gov/co/st/en/fo/wrfo/Cultural_Resources/formative_era_fremont.html |title=Formative Era/Fremont Culture |publisher=U.S. Bureau of Land Management |date=August 31, 2009 |access-date=April 9, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120623051811/http://www.blm.gov/co/st/en/fo/wrfo/Cultural_Resources/formative_era_fremont.html |archive-date=June 23, 2012}}{{cite web |url=http://www.nps.gov/dino/historyculture/fremont-culture.htm |title=Fremont Culture |publisher=U.S. National Park Service |work=Dinosaur National Monument |access-date=April 9, 2012 |archive-date=August 15, 2012 |archive-url=https://web.archive.org/web/20120815052653/http://www.nps.gov/dino/historyculture/fremont-culture.htm |url-status=live }}{{cite web |last=Vess |first=Deborah |url=http://www.faculty.de.gcsu.edu/~dvess/ids/amtours/anawciv.htm |title=The Anasazi |publisher=Georgia College and State University |archive-url=https://web.archive.org/web/20110608070125/http://www.faculty.de.gcsu.edu/~dvess/ids/amtours/anawciv.htm |archive-date=June 8, 2011}}

While there is much evidence of ancient habitation along the Colorado River, including stone dwellings, petroglyphs and pottery in places such as Glen Canyon,{{cite news|url=https://www.knau.org/knau-and-arizona-news/2022-05-12/archaeological-sites-once-thought-lost-under-lake-powell-reappear-as-water-drops|title=Archaeological sites once thought lost under Lake Powell reappear as water drops|work=Arizona Public Radio|author=Sevigny, Melissa|date=May 12, 2022|accessdate=August 11, 2024}} the first major agriculture-based societies arose a significant distance from the river. The Puebloan people built many multi-story pueblos or "great houses", and developed complex distribution systems to supply drinking and irrigation water in Chaco Canyon in northwestern New Mexico{{cite web |url=http://cpluhna.nau.edu/Change/waterdevelopment2.htm |title=Water Development, Extraction, and Diversion |page=2 |work=Land Use History of North America: Colorado Plateau |publisher=Northern Arizona University |access-date=October 21, 2014 |url-status=dead |archive-url=https://web.archive.org/web/20150214234430/http://cpluhna.nau.edu/Change/waterdevelopment2.htm |archive-date=February 14, 2015}} and Mesa Verde in southwest Colorado.{{cite web|url=https://www.nps.gov/meve/learn/historyculture/upload/ancestral_pueblo_people_2018_508_01-24-18-2.pdf|title=Ancestral Pueblo People and Their World|publisher=U.S. National Park Service|date=|accessdate=August 20, 2024}} The Hohokam, present in the modern Phoenix area since about 0 CE, experienced prolific growth around 600–700 CE as they constructed a large system of irrigation canals making use of the Salt River. Both civilizations supported large populations at their height, with 6,000–15,000 in Chaco Canyon{{sfn|Nobles|1998|p = 26}} and as many as 30,000–200,000 Hohokam.{{sfn|Logan|2006|pp = 21–22}}

Puebloan and Hohokam settlements were abruptly abandoned in the 1400s CE, due both to over-exploitation of natural resources such as timber, and severe drought that made it impossible to maintain irrigation systems.{{cite web |url=http://tenthmil.com/mission/timeline/ancient_forest_management_in_the_chaco_canyon |title=Ancient Forest Management in the Chaco Canyon – From 600 AD to 1300 AD |publisher=Tenthmil |archive-url=https://web.archive.org/web/20101224005421/http://tenthmil.com/mission/timeline/ancient_forest_management_in_the_chaco_canyon |archive-date=December 24, 2010}}{{cite journal |title=Sensitivity of the Colorado Plateau to Change: Climate, Ecosystems and Society |journal=Ecology and Society |volume=13 |page=28 |year=2008 |author1=Schwinning, Susan |author2=Belnap, Jayne |author3=Bowling, David R. |author4=Ehleringer, James R. |name-list-style=amp |issue=2 |doi=10.5751/ES-02412-130228 |doi-access=free |hdl=10535/2863 |hdl-access=free}} Many Puebloans migrated east to the Rio Grande Valley, while others persisted in smaller settlements on the Colorado Plateau. Puebloan descendants include the Hopi, Zuni, Laguna and Acoma peoples of modern Arizona and New Mexico. O'odham peoples, including the Akimel O'odham (Pima) and Maricopa who continue to live in southern Arizona, are believed to be descended from the Hohokam.{{cite web |url=http://phoenix.gov/recreation/arts/museums/pueblo/about/visitorinfo/materials/dfdisappearance.html |title=Desert Farmers at the River's Edge: The Hohokam and Pueblo Grande |author1=Andrews, John P. |author2=Bostwick, Todd W. |publisher=City of Phoenix |work=Pueblo Grande Museum Archaeological Park |access-date=April 10, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120508080438/http://phoenix.gov/recreation/arts/museums/pueblo/about/visitorinfo/materials/dfdisappearance.html |archive-date=May 8, 2012}}{{cite web |url=http://www.srpmic-nsn.gov/history_culture/pimapast.asp |title=A Pima Past |publisher=Salt River Pima-Maricopa Indian Community |author=Shaw, Anne Moore |access-date=April 10, 2012 |archive-url=https://web.archive.org/web/20130120034144/http://www.srpmic-nsn.gov/history_culture/pimapast.asp |archive-date=January 20, 2013 |url-status=dead}}

The lower Colorado River valley was inhabited for thousands of years by numerous tribes of the Patayan cultures, many of which belong to the Yuman-Cochimi language group. These include the Walapai, Havasupai and Yavapai in the Grand Canyon region; the Mohave, Halchidhoma, Quechan, and Halyikwamai along the Colorado River between Black Canyon and the Mexican border, and the Cocopah around the Colorado River Delta. The Chemehuevi (a branch of the Southern Paiute) and the Kumeyaay inhabited the desert to the river's west.{{cite web|url=https://open.uapress.arizona.edu/read/2d8a8c6f-6ac9-42c7-948b-c6e915ead695/section/76b1ffac-3d68-4022-bfee-c9dc93fe5f19|title=Yuman Antagonists: Maricopas, Quechans and Mohaves to 1857|publisher=University of Arizona Press|work=Massacre on the Gila: An Account of the Last Major Battle Between American Indians, with Reflections on the Origin of War|author=Kroeber, Clifton B. and Fontana, Bernard L.|accessdate=August 20, 2024}} Those living along the lower Colorado River depended more on fishing and floodplain agriculture than on irrigation, and mostly did not live in permanent settlements.{{cite web |url=http://www.nrcprograms.org/site/PageServer?pagename=swirc_res_ca_mohave |title=California: Mohave |publisher=Southwest Indian Relief Council |access-date=April 10, 2012 |archive-date=December 13, 2012 |archive-url=https://web.archive.org/web/20121213172312/http://www.nrcprograms.org/site/PageServer?pagename=swirc_res_ca_mohave |url-status=live }} The site of modern-day Yuma has been an important river crossing since ancient times, as the channel here is much narrower compared to the expansive, swampy river bottoms to the north and south, and enabled the expansion of trade to the Pima and Maricopa in the east and coastal California tribes in the west.

Image:Navajo Woman and Infant, Canyon de Chelle, Arizona (Canyon de Chelly National Monument), 1933 - 1942 - NARA - 519947.jpg woman and child, photographed by Ansel Adams, c. 1944]]

The Navajo (Diné) began migrating into the Colorado River Basin around 1000–1500 CE, and eventually exercised influence over much of the Colorado Plateau. Originally hunter-gatherers, they acquired knowledge of farming from the Puebloans and adopted a more sedentary lifestyle over time, making extensive use of irrigation in their settlements.{{cite web |url=http://www.ics.uci.edu/~aisi/97_aisics/people/jsells/assign5.html |title=Dine History and Facts |publisher=Donald Bren School of Information and Computer Sciences |location=University of California, Irvine |access-date=April 10, 2012 |url-status=dead |archive-url=https://archive.today/20121211142719/http://www.ics.uci.edu/~aisi/97_aisics/people/jsells/assign5.html |archive-date=December 11, 2012}}{{cite web |url=http://www.navajobusiness.com/pdf/FstFctspdf/A%20Brief%20History.pdf |title=The Navajo: A Brief History |publisher=The Navajo Nation Division of Economic Development |access-date=April 10, 2012 |archive-date=September 4, 2012 |archive-url=https://web.archive.org/web/20120904050849/http://navajobusiness.com/pdf/FstFctspdf/A%20Brief%20History.pdf |url-status=live }} The Navajo gradually displaced Hopi settlements as they expanded into northern Arizona after the 1500s. Navajo Mountain and Rainbow Bridge in the Glen Canyon area came to hold particular religious significance for the Navajo, and the nearby confluence of the Colorado and San Juan River is regarded as the birthplace of clouds and rain.{{cite web|url=https://muse.jhu.edu/pub/187/oa_monograph/chapter/202309|title=Navajo Mountain Religion: Rainbow Bridge through Indian Eyes|author=Hassell, Hank|publisher=Johns Hopkins University|year=2001|accessdate=August 20, 2024}}

The Ute also became established in the Colorado Plateau around 1500 CE, although they had inhabited more northerly parts of the Colorado basin (modern Wyoming and northern Colorado) since at least 0 CE.{{cite web |url=http://cpluhna.nau.edu/People/ute_indians.htm |title=Ute |publisher=Northern Arizona University |work=Land Use History of North America |access-date=April 9, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120205112023/http://www.cpluhna.nau.edu/People/ute_indians.htm |archive-date=February 5, 2012}}{{sfn|Benke|Cushing|p=486|2005}} They are the first known inhabitants of this part of the Rocky Mountains, and made use of an extensive network of trails crisscrossing the mountains to move between summer and winter camps. The Ute were divided into numerous bands with separate territories but shared a common language and customs. The Uncompahgre or Tabeguache lived around the confluence of the upper Colorado and Gunnison Rivers, an area including the Grand Mesa; the Weenuchiu lived along the San Juan River, and the Parianuche and Yamparika lived in the Yampa, White and Duchesne River valleys.{{cite web|url=https://www.southernute-nsn.gov/history/|title=History|publisher=Southern Ute Indian Tribe|accessdate=August 20, 2024}} The Ute ranged as far as the river's headwaters; one Ute story recounts a battle with the Arapaho at Grand Lake, which they believe still hosts the spirits of the deceased.

File:La conquista del Colorado.jpg, depicts Francisco Vázquez de Coronado's 1540–1542 expedition. García López de Cárdenas can be seen overlooking the Grand Canyon.]]

Starting in the 1500s, the Spanish began to explore and colonize western North America. Francisco de Ulloa may have been the first European to see the river, when in 1536 he sailed to the head of the Gulf of California.{{cite web |url=http://www.cr.nps.gov/history/online_books/geology/publications/inf/powell/sec2.htm |title=John Wesley Powell's Exploration of the Colorado River |publisher=U.S. Geological Survey |date=March 28, 2006 |access-date=February 19, 2012 |archive-url=https://web.archive.org/web/20150405233341/http://www.cr.nps.gov/history/online_books/geology/publications/inf/powell/sec2.htm |archive-date=April 5, 2015 |url-status=dead}} In 1540 García López de Cárdenas became the first European to see the Grand Canyon, during Coronado's expedition to find the Seven Cities of Gold ("Cibola"). Cárdenas was apparently unimpressed with the canyon, greatly underestimating its size, and left in disappointment with no gold to be found.{{sfn|Axelrod|Phillips|p=4|2008}}{{sfn|Lankford|pp=100–101|2010}} In the same year Melchior Díaz explored the Colorado River's delta and named it Rio del Tizon ("fire brand river"), after seeing a practice used by the local people for warming themselves.{{cite web |author1=Flint, Richard |author2=Flint, Shirley Cushing |url=http://www.newmexicohistory.org/filedetails.php?fileID=472 |archive-url=https://web.archive.org/web/20131010042202/http://www.newmexicohistory.org/filedetails.php?fileID=472 |archive-date=October 10, 2013 |title=Diaz, Melchior |publisher=New Mexico Office of the State Historian}}

By the late 1500s or early 1600s, the Utes had acquired horses from the Spanish, and their use for hunting, trade and warfare soon became widespread among Utes and Navajo in the Colorado River basin. This conferred them a military advantage over Goshutes and Southern Paiutes that were slower to adopt horses.{{sfn|Pritzker|p=309|1998}} The Navajo also adopted a culture of livestock herding as they acquired sheep and goats from the Spanish. Juan Bautista de Anza in 1774 was the first Spaniard to reach Yuma Crossing, where he established friendly relations with the Quechan people and opened the Anza trail between Arizona and the California coast. The Spanish soon founded Mission Puerto de Purisima Concepcion and Mission San Pedro y San Pablo de Bicuner along the lower Colorado River. However, Spanish attempts to control the crossing led to the 1781 Yuma revolt, in which over 100 soldiers and colonists were killed, and the settlements were abandoned. The Quechan blocked foreigners' use of the crossing until the arrival of American mountain men and fur trappers in the 1820s.{{cite web|url=https://www.nps.gov/articles/yuma.htm|title=Arizona: Yuma Crossing National Heritage Area|publisher=U.S. National Park Service|date=April 22, 2019|accessdate=August 20, 2024}}

The name Rio Colorado first appears in 1701, on the map "Paso por Tierra a la California" published by missionary Eusebio Kino, who also determined during that time that Baja California was a peninsula, not an island as previously believed.{{sfn|Bolton|2017|pp=440}} In the 1700s and early 1800s many Spanish and American explorers believed in the existence of a Buenaventura River that ran from the Rocky Mountains to the Pacific coast.{{sfn|Gudde|Bright|p=50|2004}} In 1776, Silvestre Vélez de Escalante attached this name to the upper Green River, and a number of later maps showed this connecting to Lake Timpanogos (now Utah Lake) and flowing west to California. The Dominguez–Escalante expedition first reached the Colorado River near the junction with the Dolores River, naming the larger river "Rio San Rafael". They later forded the Colorado in southeastern Utah at Crossing of the Fathers, now submerged in Lake Powell.{{cite web|url=https://www.intermountainhistories.org/items/show/65|title=Crossing of the Fathers|author=Anthony, Alex|publisher=Intermountain Histories|date=|accessdate=August 20, 2024}}

Image:%27Noon_Day_Rest_in_Marble_Canyon%27_from_the_second_Powell_Expedition_1872.jpg's second Colorado River expedition in Marble Canyon, 1872.]]

In the 1820s, American fur trappers along the upper Green River in Wyoming (known to them as the "Seedskeedee" or variants thereof), seeking a route to export furs to the coast, surmised that this and what the Spanish called the Colorado were in fact connected. William H. Ashley made an unsuccessful attempt to navigate from the Green River to the Colorado's mouth in 1825.{{cite web | url=https://issuu.com/utah10/docs/uhq_volume55_1987_number2/s/153728 | title=Before Powell: Exploration of the Colorado River }} In 1826, Jedediah Smith arrived at the lower Colorado River, referring to it as the Seedskeedee,{{sfn|Dellenbaugh|p=60|1909}} and proceeded upstream, exploring as far as Black Canyon. During the 1830s, various fur trappers from Wyoming made it as far downstream as Cataract Canyon and Glen Canyon, but none were able to navigate the full length of the river. In 1843 John C. Frémont explored the Great Basin and conclusively determined no Buenaventura River flowed west to California; thus, the direction of river flow must be southwest.{{sfn|Rolle|p=54|1999}}

By the early 19th century, the stretch of the Colorado above the confluence of the Green River at Cataract Canyon, Utah, became known to fur trappers as the "Grand River", though the exact origin of this name is unknown. The Grand River above the confluence with the Gunnison River was also called the Bunkara River, the Blue River, or the North Fork of the Grand River until the 1870s.{{cite book | url=https://books.google.com/books?id=1hVPAQAAMAAJ&dq=bunkara+river&pg=PA856 | title=Indian Affairs: Treaties | date=1904 | publisher=U.S. Government Printing Office }} By the early 1900s the name "Grand River" had been attached to the entire stream as far as Grand Lake, which was then considered its official source.{{cite book | url=https://books.google.com/books?id=ELk9AQAAMAAJ&q=grand%20river%20colo | title=A Complete Pronouncing Gazetteer or Geographical Dictionary of the World: Containing the Most Recent and Authentic Information Respecting the Countries, Cities, Towns, Resorts, Islands, Rivers, Mountains, Seas, Lakes, Etc., in Every Portion of the Globe | date=1906 | publisher=J. B. Lippincott Company }} Although the Grand River was renamed the Colorado in 1921, its name survives in numerous places such as Grand County and Grand Junction, Colorado.{{cite news|url=https://www.gjsentinel.com/news/western_colorado/whats-in-a-name-grand-junction-and-the-rivers-that-run-through-it/article_608fc78e-0a02-11ed-b9f8-4b374828fc21.html|title=What's in a Name: Grand Junction and the rivers that run through it|author=Gemaehlich, Tammy|work=The Grand Junction Daily Sentinel|date=September 4, 2023|accessdate=August 17, 2024}}

In 1848 the U.S. Army established Fort Yuma, creating the first permanent U.S. settlement along the river. This served as a military garrison and supply point for settlers headed to California along the Southern Emigrant Trail. Due to the arduous task of ferrying supplies overland, the schooner Invincible attempted to bring supplies up the river but was thwarted by the delta's strong tides. Steamboats were brought to the river, starting in 1852 with the sidewheeler Uncle Sam, whose first voyage from the Gulf to Yuma took fifteen days.{{Cite web |url=https://books.google.com/books?id=7I8xAQAAMAAJ&pg=PA14 |title=Overland Monthly and Out West Magazine |date=August 11, 1895 |publisher=A. Roman and Company |via=Google Books |access-date=August 31, 2016 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301110641/https://books.google.com/books?id=7I8xAQAAMAAJ&pg=PA14#v=onepage&q&f=false |url-status=live }}{{rp|15}} Exploration by steamboat soon advanced upriver. In 1857, George A. Johnson in the General Jesup was able to reach Pyramid Canyon, over {{convert|300|mi|km}} north of Fort Yuma.{{rp|16–17,19}}{{Cite web |url=https://oac.cdlib.org/ark:/13030/kt7p30228k/?order=2&brand=calisphere |title=Autobiography and Reminiscence of George Alonzo Johnson, San Diego, 1901. |website=oac.cdlib.org |access-date=August 11, 2022 |archive-date=August 3, 2020 |archive-url=https://web.archive.org/web/20200803144014/https://oac.cdlib.org/ark:/13030/kt7p30228k/?order=2&brand=calisphere |url-status=live }} He was followed by Lt. Joseph Christmas Ives who used a specially built shallow-draft steamboat, Explorer, to reach Black Canyon, where Hoover Dam stands today.{{Cite web |url=https://books.google.com/books?id=1xIOAAAAQAAJ |title=Report Upon the Colorado River of the West |first=Joseph C. |last=Ives |date=August 11, 1861 |publisher=U.S. Government Printing Office |via=Google Books}}{{rp|Part 1, 85–87}} Having set out to determine the river's suitability as a navigation route, Ives remarked: "Ours has been the first, and will doubtless be the last, party of whites to visit this profitless locality. It seems intended by nature that the Colorado River, along the greater portion of its lonely and majestic way, shall be forever unvisited and undisturbed."{{sfn|Dellenbaugh|p=170|1909}}{{sfn|Schmidt|p=12|1993}}

Image:First_Powell_Expedition_1869_USGS_Pioh120.jpg

The last part of the Colorado River to be surveyed was the Grand Canyon itself. In 1869, John Wesley Powell with nine men set out on an expedition from Green River Station, Wyoming. They were the first part of non-natives to travel the length of the Grand Canyon, and the first to successfully travel by boat from the upper Green River to the lower Colorado.{{sfn|Dolnick|2002|p=5}}{{sfn|Leuchtenburg|2000|p=360}} Powell led a second expedition in 1871, with financial backing from the U.S. government,

{{cite web |url=http://3dparks.wr.usgs.gov/3Dcanyons/html/glencanyon.htm |title=Historic 3D Photographs of the Second Powell Expedition (1871–1872) |publisher=U.S. Geological Survey |work=Geology of National Parks |date=February 13, 2012 |access-date=February 20, 2012 |archive-url=https://web.archive.org/web/20120311104946/http://3dparks.wr.usgs.gov/3Dcanyons/html/glencanyon.htm |archive-date=March 11, 2012 |url-status=dead}}

and continued to conduct geographical and botanical surveys across the region until the 1890s.{{cite web|url=https://siarchives.si.edu/collections/auth_exp_fbr_eace0023|title=The J.W. Powell Survey (1871-1894)|publisher=Smithsonian Institution Archives|accessdate=August 20, 2024}} Another Grand Canyon river expedition was led in 1889–1890 by Robert Brewster Stanton to survey a route for a proposed railroad through the canyon, which was never built.{{cite journal|url=http://www.nativefishlab.net/library/textpdf/16614.pdf|title=The Colorado River Railroad Survey|author=Hekkers, Jim|journal=Colorado Outdoors|year=1980|accessdate=August 17, 2024}}

{{quotation|

We are now ready to start on our way down the Great Unknown. Our boats, tied to a common stake, are chafing each other, as they are tossed by the fretful river. They ride high and buoyant, for their loads are lighter than we could desire. We have but a month's rations remaining… The lighting of the boats has this advantage: they will ride the waves better, and we shall have little to carry when we make a portage.

We are three-quarters of a mile in the depths of the earth, and the great river shrinks into insignificance, as it dashes its angry waves against the walls and cliffs, that rise to the world above; they are but puny ripples, and we but pigmies, running up and down the sands, or lost among the boulders.

We have an unknown distance yet to run; an unknown river yet to explore. What falls there are, we know not; what rocks beset the channel, we know not; what walls rise over the river, we know not; Ah, well! we may conjecture many things. The men talk as cheerfully as ever; jests are bandied about freely this morning; but to me the cheer is somber and the jests are ghastly.|John Wesley Powell's journal, August 1869{{sfn|Dolnick|2002|p=238}}}}

{{see also|Steamboats of the Colorado River}}

Image:Fort_Yuma_California_1875.jpg of Fort Yuma, c. 1875]]

In 1858, gold was discovered on the Gila River east of Yuma, then along the Colorado River at El Dorado Canyon, Nevada and La Paz, Arizona.{{cite web|url=https://www.intermountainhistories.org/items/show/400|title=Nelson, Nevada Ghost Town and El Dorado Canyon|publisher=Intermountain Histories|author=Keeler, Preston|date=|accessdate=August 20, 2024}}{{cite news|url=https://www.azcentral.com/story/travel/arizona/road-trips/2018/09/10/arizona-mining-history-wild-west/953080002/|title=Arizona's mining history: Danger for many, riches for a few|author=Johnson, Weldon B.|work=The Arizona Republic|date=September 10, 2018}} As prospectors and settlers entered the region, they became involved in skirmishes with the Mohave, spurring U.S. Army expeditions that culminated in the 1859 Battle of the Colorado River which concluded the Mohave War.{{sfn|Kessel|Wooster|p=217|2005}} In the 1870s the Mohave were moved to the Fort Mohave and Colorado River reservations.{{cite web |url=http://public.csusm.edu/loc/rezinfo/ftmojave/index.htm |archive-url=https://web.archive.org/web/20130514234037/http://public.csusm.edu/loc/rezinfo/ftmojave/index.htm |archive-date=May 14, 2013 |title=Fort Mojave Reservation |publisher=California State University San Marcos |work=Tribal Library Census and Needs Assessment |date=June 12, 2001}}{{cite web |url=http://www.crit-nsn.gov/crit_contents/about/ |title=About the Mohave, Chemehuevi, Hopi and Navajo Tribes |publisher=Colorado River Indian Tribes |year=2009 |access-date=April 10, 2012 |archive-date=August 12, 2021 |archive-url=https://web.archive.org/web/20210812031022/https://crit-nsn.gov/crit_contents/about/ |url-status=live }} Chemehuevi and later some Hopi and Navajo peoples were also moved to the Colorado River reservation, where they today form the Colorado River Indian Tribes.{{cite web |url=http://www.indiancountryextension.org/sites/indiancountryextension.org/files/publications/files/u6/CRIT%20and%20Extension%20Programs.pdf |title=The Colorado River Indian Tribes (C.R.I.T.) Reservation and Extension Programs |publisher=Indian Country Extension |work=University of Arizona College of Agriculture and Life Sciences |date=October 2008 |access-date=April 10, 2012 |archive-url=https://web.archive.org/web/20130120034152/http://www.indiancountryextension.org/sites/indiancountryextension.org/files/publications/files/u6/CRIT%20and%20Extension%20Programs.pdf |archive-date=January 20, 2013 |url-status=dead}}

As the American frontier expanded into the Colorado Plateau, an effort to expel the Navajo from the Four Corners region was begun by General James Henry Carleton, who in 1864 enlisted mountain man Kit Carson to lead a campaign against the Navajo. Carson, with the help of the Navajo's Ute enemies, captured more than 8,000 Navajo and forcibly marched them to Fort Sumner, New Mexico. Hundreds died during what is now known as the Long Walk and while enduring appalling conditions at Fort Sumner. After the failure of the Army to maintain the reservation there, the Treaty of Bosque Redondo established the Navajo Nation in the Four Corners, where the Navajo were allowed to return in 1868.{{cite web |url=http://www.ihs.gov/navajo/index.cfm?module=nao_navajo_nation |title=Navajo Nation |publisher=Indian Health Service |access-date=April 10, 2012 |archive-date=June 23, 2012 |archive-url=https://web.archive.org/web/20120623162828/http://www.ihs.gov/Navajo/index.cfm?module=nao_navajo_nation |url-status=live }}{{cite web |url=http://reta.nmsu.edu/modules/longwalk/lesson/document/treaty.htm |archive-url=https://web.archive.org/web/20020311010532/http://reta.nmsu.edu/modules/longwalk/lesson/document/treaty.htm |url-status=dead |archive-date=March 11, 2002 |title=Treaty Between the United States of America and the Navajo Tribe of Indians |publisher=New Mexico State University |work=Historic Documents |year=1868 |access-date=April 10, 2012}}{{cite web |url=http://www.navajobusiness.com/fastFacts/Overview.htm |title=Navajo Nation – Facts at a Glance |publisher=The Navajo Nation |year=2004 |access-date=April 10, 2012 |archive-date=March 8, 2012 |archive-url=https://web.archive.org/web/20120308193308/http://www.navajobusiness.com/fastFacts/Overview.htm |url-status=live }}

Gold and silver were also discovered in the Upper Basin, beginning with the 1859 Blue River strike that led to the founding of Breckenridge, Colorado.{{sfn|Brown|pp=52–53|1972}} Up until the 1860s, southwestern Colorado had remained relatively untouched by U.S. westward expansion, as the Americans had recognized Ute sovereignty by treaty. Following the 1861 carving out of Colorado Territory and further mineral strikes including Ouray and Telluride,{{sfn|Casey|p=251|2007}}{{sfn|Lindberg|pp=134–135|2009}} Ute leaders were coerced into signing the 1873 Brunot Agreement, in which they lost rights to most of their land. A flood of mineral prospecting and settlement ensued in western Colorado.{{cite web|url=https://www.nps.gov/parkhistory/online_books/blm/co/10/chap5.htm|title=Chapter 5: The Utes in Southwestern Colorado: A Confrontation of Cultures|publisher=U.S. National Park Service|work=Frontier in Transition: A History of Southwestern Colorado|access-date=2017-05-15}} By 1881, the Army had driven out the remaining pockets of Ute resistance on the Western Slope, officially opening the Grand River country to settlement, and the town of Grand Junction was incorporated a year later.{{cite news|url=https://www.postindependent.com/news/local/gj-history-grand-junction-town-founder-george-a-crawford-a-witness-to-early-american-history/|title=GJ HISTORY: Grand Junction town founder George A. Crawford — A witness to early American history|work=The Post-Independent|date=September 26, 2013|accessdate=August 20, 2024}} The Denver and Rio Grande Western Railroad (D&RGW) quickly expanded into this area to serve mining boomtowns, crossing the Rockies to the south via the Black Canyon of the Gunnison River. By 1883 the railroad had reached Grand Junction, and a spur up the Colorado to Glenwood Springs was completed in 1887.{{cite web | url=https://www.postindependent.com/news/glenwood-canyon-time-travel-a-journey-back-through-the-history-of-travel-through-the-grand-river-canon-passage/ | title=Glenwood Canyon time travel: A journey back through the history of travel through the 'Grand River Cañon' passage | date=August 15, 2021 }}

In Arizona and Utah Territories, many early settlers were Mormons fleeing religious persecution in the Midwest. Mormons founded agricultural colonies at Fort Santa Clara in 1855 and St. Thomas, now flooded under Lake Mead, in 1865.{{cite web|url=https://www.nps.gov/lake/learn/nature/st-thomas-nevada.htm#:~:text=St.%20Thomas%20was%20founded%20in%201865%20by%20Mormon,flowed%20to%20the%20Colorado%20River%2C%2022%20miles%20south.|title=St. Thomas, Nevada|publisher=U.S. National Park Service|date=December 15, 2022|accessdate=August 20, 2024}} Stone's Ferry, crossing the Colorado at the mouth of the Virgin River, enabled shipping of their produce by wagon to gold mining districts further south. Although the Mormons abandoned St. Thomas in 1871, a salt-mining industry persisted here, and steamboats operated up to nearby Rioville into the 1880s.{{sfn|Glass|Glass|pp=162–163|1983}}{{Cite book |author=Richard E. Lingenfelter |url=http://www.ansac.az.gov/UserFiles/PDF/08182014/X028_FMIBurtellLingenfelterSteamboats/FMI%20Lingenfelter%20Steamboats/Steamboats%20on%20the%20Colorado%20River%201852-1916.pdf |archive-url=https://web.archive.org/web/20160118031332/http://www.ansac.az.gov/UserFiles/PDF/08182014/X028_FMIBurtellLingenfelterSteamboats/FMI%20Lingenfelter%20Steamboats/Steamboats%20on%20the%20Colorado%20River%201852-1916.pdf |archive-date=January 18, 2016 |url-status=dead |title=Steamboats on the Colorado River, 1852–1916 |publisher=University of Arizona Press |location=Tucson |year=1978}}{{rp|78}} In 1879 a group of Mormon settlers made their way to southeastern Utah, blasting the precarious Hole in the Rock Trail to cross the Colorado River at Glen Canyon, subsequently establishing the community of Bluff.{{cite web|url=https://www.nps.gov/glca/learn/historyculture/holeintherock.htm|title=Hole-in-the-Rock|publisher=U.S. National Park Service|date=July 28, 2023|accessdate=August 20, 2024}} Due to the dry climate, these settlements depended heavily on irrigation. In central Arizona, settlers uncovered and re-established canals previously used by the Hohokam.{{cite web |url=http://www.waterhistory.org/histories/hohokam2/ |title=Hohokam Legacy: Desert Canals |website=WaterHistory.org |author=Howard, Jerry B. |access-date=April 9, 2012 |archive-url=https://web.archive.org/web/20120124025948/http://www.waterhistory.org/histories/hohokam2/ |archive-date=January 24, 2012 |url-status=dead}}{{cite web |author=Williams, O.A. |url=http://uair.arizona.edu/system/files/usain/download/azu_e9791_1937_67_w.pdf |title=Settlement and Growth of the Gila Valley as a Mormon Colony, 1879–1900 |publisher=University of Arizona |year=1937 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130120034142/http://uair.arizona.edu/system/files/usain/download/azu_e9791_1937_67_w.pdf |archive-date=January 20, 2013}}

Image:File-Historic_photograph_of_ferryboat_at_Lees_Ferry._John-D._Lee_established_the_first_ferry_at_the_confluence_of_the-Colorado_and_(698b6643-1dc1-4738-866e-a38aa7046253).jpg

Following tensions between Mormon settlers and the U.S. government in the Utah War, a local militia including John D. Lee perpetrated the 1857 Mountain Meadows Massacre, in which 120 non-Mormon settlers were killed. Fearing retribution, Lee moved in 1870 to the remote Pahreah Crossing in Arizona, where he took over a ferry first established in 1864 by Jacob Hamblin.{{cite web |url=http://www.nps.gov/glca/historyculture/leesferryhistory.htm |title=Lees Ferry History |publisher=U.S. National Park Service |work=Glen Canyon National Recreational Area |date=August 11, 2006 |access-date=February 20, 2012 |archive-date=April 14, 2012 |archive-url=https://web.archive.org/web/20120414171025/http://www.nps.gov/glca/historyculture/leesferryhistory.htm |url-status=live }} This, the only river crossing for hundreds of miles not hemmed in by vertical canyon walls, became known as Lee's Ferry. While Lee was tried and subsequently executed in 1877, the ferry remained a major transportation link until the Navajo Bridge was completed nearby in 1928, rendering the ferry obsolete.{{cite web |url=http://grandcanyonhistory.clas.asu.edu/sites_coloradorivercorridor_leesferry.html |title=Lees Ferry |publisher=Arizona State University |date=July 8, 2010 |access-date=February 20, 2012 |archive-url=https://web.archive.org/web/20120603033950/http://grandcanyonhistory.clas.asu.edu/sites_coloradorivercorridor_leesferry.html |archive-date=June 3, 2012 |url-status=dead}}

The Denver and Salt Lake Railway (D&SL), incorporated in 1902, sought to provide a more direct connection between Denver and Salt Lake City than either the transcontinental railroad through Wyoming or the D&RGW's route via Black Canyon and Durango. The D&SL completed a rail line into the upper headwaters of the Colorado River and blasted the Moffat Tunnel under the Continental Divide, but ran out of money before even reaching Utah. In 1931 the D&RGW completed the "Dotsero Cutoff" linking Glenwood Springs to the D&SL route at Bond, Colorado, finally completing the direct Denver–Salt Lake link with its acquisition of the bankrupt D&SL. The Gunnison River route was eventually abandoned in favor of the shorter Colorado River route, which today is owned by Union Pacific.{{cite web|url=https://www.nps.gov/parkhistory/online_books/blm/cultresser/co/2/chap9.htm#:~:text=In%201931%20the%20Denver%20and,Denver%20to%20Salt%20Lake%20City.|title=The "Moffat Road" and Northwestern Colorado, 1903-1948|publisher=U.S. National Park Service|work=BLM Cultural Resource Series: An Isolated Empire: A History of Northwest Colorado|date=|accessdate=August 20, 2024}}{{cite web|url=https://www.nps.gov/blca/learn/historyculture/railroad.htm|title=Black Canyon of the Gunnison: Narrow Gauge Railroad|publisher=U.S. National Park Service|date=December 2, 2022|accessdate=August 20, 2024}}

As late as 1921, the Colorado River upstream from the confluence with the Green River in Utah was still known as the Grand River. For over a decade, U.S. Representative Edward T. Taylor of Colorado had petitioned the Congressional Committee on Interstate and Foreign Commerce to rename the Grand River as the Colorado River.{{citation |url=http://www.riversimulator.org/Resources/LawOfTheRiver/HearingToRenameGrandRiverColorado1921.pdf |work=Hearing Before the Committee on Interstate and Foreign Commerce of the House of Representatives, Sixty Sixth Congress, Third Session, on HJ 460 |publisher=Government Printing Office |title=Renaming the Grand River, Colo. |date=February 18, 1921 |access-date=May 16, 2023 |archive-date=August 5, 2021 |archive-url=https://web.archive.org/web/20210805160830/http://www.riversimulator.org/Resources/LawOfTheRiver/HearingToRenameGrandRiverColorado1921.pdf |url-status=live }}{{cite news |author=Colorado River Water Conservation District |date=December 23, 2003 |title=Many Years Ago, the Colorado River Was Just Grand |url=http://www.summitdaily.com/article/20031223/OPINION/312230302 |archive-url=https://web.archive.org/web/20110430050158/http://www.summitdaily.com/article/20031223/OPINION/312230302 |archive-date=April 30, 2011 |access-date=January 5, 2008 |publisher=SummitDaily}} Representatives from Wyoming, Utah, and the United States Geological Survey objected, noting that the Green River was longer and drained a larger area. Taylor argued that the Grand River should be considered the main stream, as it carried the larger volume of water.{{sfn|Barnes|p=104|1988}}{{refn|The average discharge of the Colorado (Grand) River at Cisco, Utah, about {{convert|97|mi|km}} upstream from the Green River confluence, is {{convert|7181|cuft/s|m3/s}}; between here and the confluence, only a few small, intermittent tributaries join the river.The Green River has an average discharge of {{convert|6048|cuft/s|m3/s}} as measured at Green River, Utah, about {{convert|117.6|mi|km}} above the confluence; below here the only major tributary is the San Rafael River, which contributes an average of {{convert|131|cuft/s|m3/s}}, resulting in a total of {{convert|6169|cuft/s|m3/s}}, still significantly lower than the discharge of the Colorado at their confluence.{{cite web |url=http://wdr.water.usgs.gov/wy2012/pdfs/09328500.2012.pdf |title=USGS Gage #09328500 on the San Rafael River near Green River, Utah |publisher=U.S. Geological Survey |work=National Water Information System |date=1910–2012 |access-date=June 22, 2013 |archive-date=September 19, 2015 |archive-url=https://web.archive.org/web/20150919112455/http://wdr.water.usgs.gov/wy2012/pdfs/09328500.2012.pdf |url-status=live }}|group=n}} Taylor felt "slighted" that the Colorado River, as named, did not begin in the state of Colorado, and "he wasn't going to let Utah or Wyoming lay claim to the river's headwaters, despite the fact that the Green River is the larger drainage basin."{{cite web|url=https://www.colorado.edu/center/west/2017/12/20/how-grand-became-colorado-and-what-it-says-about-our-relationship-nature|title=How the 'Grand' Became the 'Colorado' And What It Says About Our Relationship To Nature|publisher=University of Colorado Boulder|date=December 20, 2017|work=KUNC|author=Runyon, Luke|accessdate=August 20, 2024}} On July 25, 1921, President Warren G. Harding signed House Joint Resolution 32 - "To change the name of the Grand River in Colorado and Utah to the Colorado River."{{cite web |url=https://www.govinfo.gov/content/pkg/GPO-CRECB-1921-pt5-v61/pdf/GPO-CRECB-1921-pt5-v61.pdf |title=House Joint Resolution 32 - To change the name of the Grand River in Colorado and Utah to the Colorado River |work=Congressional Record |publisher=Sixty-seventh United States Congress |date=July 25, 1921 |page=4274 |access-date=May 29, 2023 |archive-date=May 16, 2023 |archive-url=https://web.archive.org/web/20230516224606/https://www.govinfo.gov/content/pkg/GPO-CRECB-1921-pt5-v61/pdf/GPO-CRECB-1921-pt5-v61.pdf |url-status=live }}

File:HooverDamFrontWater.jpg releasing water in 1998]]

{{See also|International Boundary and Water Commission|Dams in the Colorado River system|Colorado River Compact}}

About 40 million people depend on the Colorado River's water for agricultural, industrial and domestic needs.{{cite web |url=https://www.usbr.gov/watersmart/bsp/docs/finalreport/ColoradoRiver/CRBS_Executive_Summary_FINAL.pdf |title=Colorado River Basin Water Supply and Demand Study |publisher=U.S. Bureau of Reclamation |date=December 2012 |access-date=May 18, 2022 |archive-date=May 20, 2022 |archive-url=https://web.archive.org/web/20220520223319/https://www.usbr.gov/watersmart/bsp/docs/finalreport/ColoradoRiver/CRBS_Executive_Summary_FINAL.pdf |url-status=live }} The Colorado irrigates 5.5 million acres (2.2 million hectares) of farmland, and its hydroelectric plants produce 12 billion kilowatt hours (KWh) of hydroelectricity each year. Hydroelectricity from the Colorado is a key supplier of peaking power on the Southwest electric grid.{{cite web |url=http://www.circleofblue.org/waternews/2010/world/low-water-may-still-hoover-dam%E2%80%99s-power/ |title=Low Water May Halt Hoover Dam's Power |publisher=Circle of Blue |date=September 22, 2010 |access-date=October 8, 2012 |archive-url=https://web.archive.org/web/20130120034145/http://www.circleofblue.org/waternews/2010/world/low-water-may-still-hoover-dam%E2%80%99s-power/ |archive-date=January 20, 2013 |url-status=dead}}{{cite web |url=http://www.gcdamp.gov/keyresc/hydropower.html |title=Hydropower |publisher=Glen Canyon Dam Adaptive Management Program |date=November 18, 2010 |access-date=May 12, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120522080526/http://www.gcdamp.gov/keyresc/hydropower.html |archive-date=May 22, 2012}} Often called "America's Nile",{{cite news |author=Waterman, Jonathan |url=http://newswatch.nationalgeographic.com/2010/08/11/restoring_floods_to_grand_caynon_river/ |title=Restoring floods to America's Nile |work=National Geographic |date=August 11, 2010 |access-date=March 2, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120324005737/http://newswatch.nationalgeographic.com/2010/08/11/restoring_floods_to_grand_caynon_river/ |archive-date=March 24, 2012}} the Colorado River is so intensively managed that each drop of its water is used an average of 17 times in a single year.{{cite web |author=Nabhan, Gary Paul |url=http://www.environment.nau.edu/water/ColoradoRiver.htm |title=The Beginning and the End of the Colorado River: Protecting the Sources, Ensuring its Courses |publisher=Northern Arizona University |date=January 16, 2007 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111225000708/http://www.environment.nau.edu/water/ColoradoRiver.htm |archive-date=December 25, 2011}}{{sfn|Reisner|1993|p=120}} Southern Nevada Water Authority has called the Colorado River one of the "most controlled, controversial and litigated rivers in the world".{{cite web |url=http://www.snwa.com/ws/river_law.html |title=Colorado River Law |publisher=Southern Nevada Water Authority |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120426071547/http://www.snwa.com/ws/river_law.html |archive-date=April 26, 2012}}

In 1922, six U.S. states signed the Colorado River Compact, which divided half of the river's flow to both the Upper Basin (the drainage area above Lee's Ferry, comprising parts of Colorado, New Mexico, Utah, and Wyoming and a small portion of Arizona) and the Lower Basin (Arizona, California, Nevada, and parts of New Mexico and Utah). The Upper and Lower Basin were each allocated {{convert|7.5|e6acre.ft|km3}} of water per year, a figure believed to represent half of the river's annual flow at Lee's Ferry.{{cite web |url=http://www.usbr.gov/lc/region/pao/pdfiles/crcompct.pdf |title=Colorado River Compact, 1922 |publisher=U.S. Bureau of Reclamation |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120915093352/http://www.usbr.gov/lc/region/pao/pdfiles/crcompct.pdf |archive-date=September 15, 2012}} The allotments operated under the premise that approximately 17.5 million acre-feet of water flowed through the river annually.{{Cite web |date=2022-08-15 |title=Big changes are coming for the Colorado River soon—and they could get messy |url=https://www.nationalgeographic.com/environment/article/big-changes-are-coming-for-the-colorado-river-soonand-they-could-get-messy |archive-url=https://web.archive.org/web/20220815162514/https://www.nationalgeographic.com/environment/article/big-changes-are-coming-for-the-colorado-river-soonand-they-could-get-messy |url-status=dead |archive-date=August 15, 2022 |access-date=2022-08-16 |website=Environment |language=en}}

Arizona initially refused to ratify the compact because it feared that California would take too much of the lower basin allotment. In 1944 a compromise was reached in which Arizona was allocated {{convert|2.8|e6acre.ft|km3}}, but with the caveat that California's {{convert|4.4|e6acre.ft|km3|adj=on}} allocation was prioritized during drought years. These and nine other decisions, compacts, federal acts and agreements made between 1922 and 1973 constitute what is now known as the Law of the River.{{cite web |author=Anderson, Larry |url=http://www.water.utah.gov/interstate/thecoloradoriverart.pdf |title=Utah's Perspective: The Colorado River |publisher=Utah Division of Water Resources |date=May 2002 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20140409152252/http://www.water.utah.gov/interstate/thecoloradoriverart.pdf |archive-date=April 9, 2014}}{{cite web |url=http://www.usbr.gov/lc/region/g1000/lawofrvr.html |title=The Law of the River |publisher=U.S. Bureau of Reclamation |date=March 2008 |access-date=February 18, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120626204917/http://www.usbr.gov/lc/region/g1000/lawofrvr.html |archive-date=June 26, 2012}}

In 1944, a treaty between the U.S. and Mexico allocated {{convert|1.5|e6acre.ft|km3}} of Colorado River water to Mexico each year.{{cite web |url=http://www.ibwc.gov/Files/1944Treaty.pdf |title=Utilization of Waters of the Colorado and Tijuana Rivers and of the Rio Grande: Treaty Between the United States of America and Mexico |publisher=International Boundary and Water Commission |date=February 3, 1944 |access-date=February 20, 2012 |archive-date=September 15, 2012 |archive-url=https://web.archive.org/web/20120915121837/http://www.ibwc.gov/Files/1944Treaty.pdf |url-status=dead}} Morelos Dam was constructed in 1950 to enable Mexico to utilize its share of the river. Water allocated to Mexico from the Colorado River is regulated by the International Boundary and Water Commission, which also apportions waters from the Rio Grande between the two countries.{{cite web |url=https://www.usbr.gov/lc/yuma/facilities/dams/yao_dams_map.html |title=Dams Along the Lower Colorado River |publisher=U.S. Bureau of Reclamation |date= |access-date=May 18, 2022 |archive-date=May 17, 2022 |archive-url=https://web.archive.org/web/20220517135610/https://www.usbr.gov/lc/yuma/facilities/dams/yao_dams_map.html |url-status=live }}

Image:GRAND DITCH OVERVIEW, VIEWING SOUTH - Grand Ditch, Baker Creek to LaPoudre Pass Creek, Grand Lake, Grand County, CO HAER COLO,25-GRLK.V,2-1.tifFile:Little Oso Diversion Dam.jpg and transports it through a series of tunnels into the Rio Grande basin as part of the San Juan–Chama Project.]]

Large-scale development of Colorado River water supplies started in the late 19th century, at the river's headwaters in La Poudre Pass. The Grand Ditch, directing runoff from the river's headwaters across the Continental Divide to arid eastern Colorado, was considered an engineering marvel when completed in 1890.{{cite web |url=http://www.roaringfork.org/pub/collaborative/GDII%20Appendix%20I%20-%20Transmountain%20Diversion%20Projects.pdf |title=Appendix I: The Past, Present and Future of Transmountain Diversion Projects |publisher=Roaring Fork Conservancy |work=Roaring Fork Watershed Plan |access-date=March 18, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130120034135/http://www.roaringfork.org/pub/collaborative/GDII%20Appendix%20I%20-%20Transmountain%20Diversion%20Projects.pdf |archive-date=January 20, 2013}} This was the first of twenty-four "transmountain diversions" constructed to draw water across the Rocky Mountains as the Front Range corridor increased in population.{{cite web |url=http://www.crwcd.org/media/uploads/20110719-policies_TMDs.pdf |title=Transmountain Water Diversions |publisher=Colorado River District |date=July 2011 |access-date=March 18, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111201032049/http://www.crwcd.org/media/uploads/20110719-policies_TMDs.pdf |archive-date=December 1, 2011}} These diversions draw water from the upper Colorado and its tributaries into the South Platte River, Arkansas River and Rio Grande basins.{{cite book |publisher=Water Education Colorado |title=Citizen's Guide to Colorado's Transbasin Diversions |year=2014 |url=https://issuu.com/cfwe/docs/cfwe_cgtb_web |access-date=May 19, 2022 |archive-date=May 19, 2022 |archive-url=https://web.archive.org/web/20220519013450/https://issuu.com/cfwe/docs/cfwe_cgtb_web |url-status=live }} Today, about 80 percent of Colorado's population lives on the eastern slope of the Rockies, while 80 percent of precipitation falls on the western slope.{{cite web |url=http://www.roaringfork.org/your-watershed/watershed-facts/transmountain-diversions/ |title=Transmountain Diversions |publisher=Roaring Fork Conservancy |date= |access-date=May 18, 2022 |archive-date=May 18, 2022 |archive-url=https://web.archive.org/web/20220518060240/http://www.roaringfork.org/your-watershed/watershed-facts/transmountain-diversions/ |url-status=live }}

While first planned at the same time as the Grand Ditch, construction on the Colorado–Big Thompson Project (C-BT) did not begin until the 1930s. Today, the C-BT is the largest of the transmountain diversions, delivering {{convert|230000|acre feet|m3}} per year from the Colorado River to cities north of Denver.{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=Colorado-Big+Thompson+Project |title=Colorado-Big Thompson Project |publisher=U.S. Bureau of Reclamation |date=October 18, 2011 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111112085341/http://www.usbr.gov/projects/Project.jsp?proj_Name=Colorado-Big%20Thompson%20Project |archive-date=November 12, 2011}} Numerous other projects followed, with the largest including the Roberts Tunnel, which delivers water from the Blue River to the city of Denver,{{cite web |url=http://blueriverwatershed.org/water-quantity/moving-water-in-the-brw/ |title=Moving Water in the Watershed |publisher=Blue River Watershed Group |access-date=May 18, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120329184230/http://blueriverwatershed.org/water-quantity/moving-water-in-the-brw/ |archive-date=March 29, 2012}}{{cite news |author=Wahlstrom, Ernest E. |title=History of Geologic Investigations, Engineering Design, and Construction Methods of the Harold D. Roberts Tunnel, Colorado |publisher=U.S. Geological Survey |work=Geological Survey Professional Paper 831–A |url=http://pubs.usgs.gov/pp/0831a/report.pdf |year=1974 |access-date=May 18, 2012 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071446/https://pubs.usgs.gov/pp/0831a/report.pdf |url-status=live }} and the Fryingpan–Arkansas Project, which diverts water from the Fryingpan River to the Arkansas River basin.{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=Fryingpan-Arkansas%20Project |title=Fryingpan–Arkansas Project |publisher=U.S. Bureau of Reclamation |date=May 10, 2011 |access-date=May 18, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20140827213909/http://www.usbr.gov/projects/Project.jsp?proj_Name=Fryingpan-Arkansas%20Project |archive-date=August 27, 2014}}

Combined, the transmountain diversions draw about {{convert|580000|acre feet|m3}} of water per year out of the Colorado River basin. Historically, most of the water has been used for irrigation, although water usage is increasing for urban water supply and for recreational purposes such as snowmaking and increasing Eastern Slope streamflows for boating and fishing. Denver Water receives about 50 percent of its supply from the Colorado River basin. However, diversions have caused environmental harm to the upper Colorado River system by reducing streamflows in many tributaries. A number of reservoirs have been built to offset the impact of transmountain diversions by storing water for dry season release on the Western Slope, including Williams Fork Reservoir in 1959{{cite web |url=https://www.denverwater.org/recreation/williams-fork-resevoir |title=Williams Fork Reservoir |publisher=Denver Water |date= |access-date=May 18, 2022 |archive-date=May 22, 2022 |archive-url=https://web.archive.org/web/20220522195133/https://www.denverwater.org/recreation/williams-fork-resevoir |url-status=live }} and Wolford Mountain Reservoir in 1996.{{cite web |url=https://www.coloradoriverdistrict.org/wolford-mountain-reservoir-2/ |title=Wolford Mountain Reservoir |publisher=Colorado River District |date= |access-date=May 18, 2022 |archive-date=May 20, 2022 |archive-url=https://web.archive.org/web/20220520101522/https://www.coloradoriverdistrict.org/wolford-mountain-reservoir-2/ |url-status=live }}

{{main|Alamo Canal|Salton Sea}}

The Colorado River delta region in Mexico became a favored place for Americans to invest in agriculture in the late 19th century when Mexican President Porfirio Díaz welcomed foreign capital to develop the country. The Colorado River Land Company, formed by Los Angeles Times publisher Harry Chandler, his father-in-law Harrison Gray Otis, and others, developed the Mexicali Valley in Baja California as a thriving land company. The company headquarters was nominally based in Mexico, but its real headquarters was in Los Angeles, California. Land was leased mainly to Americans who were required to develop it. Colorado River water was used to irrigate the rich soil. The company largely escaped the turmoil of the Mexican Revolution (1910–20), but in the postrevolutionary period, the Mexican government expropriated the company's land to satisfy the demand for land reform.Dwyer, John J. The Agrarian Dispute: The Expropriation of American-Owned Rural Land in Postrevolutionary Mexico. Durham: Duke University Press 2008Kerig, Dorothy P. "Yankee Enclave: The Colorado River Land Company and Mexican Agrarian Reform in Baja California, 1902-1944." PhD diss. University of California, Irvine 1988.Castillo-Muñoz, Verónica. The Other California: Land, Identities, and Politics on the Mexican Borderlands. Oakland: University of California Press 2017.

In 1900, the California Development Company (CDC) envisioned irrigating the Imperial Valley, a then dry basin on the California–Mexico border, using water from the Colorado River. Due to the valley's location below sea level, water could be diverted and allowed to flow there entirely by gravity. Engineer George Chaffey was hired to design the Alamo Canal, which split from the Colorado near Pilot Knob, California and ran south into Mexico, where it joined the Alamo River, a dry arroyo which had historically carried overflowing floodwaters from the Colorado into the Salton Sink at the bottom of Imperial Valley. The scheme worked initially; by 1903, about four thousand people lived in the valley and more than {{convert|100000|acre|ha}} of farmland had been developed.{{sfn|Billington|Jackson|Melosi|p=140|2005}}{{cite journal |author=Sperry, Robert L. |url=https://sandiegohistory.org/journal/1975/january/imperial-2/ |title=When The Imperial Valley Fought For Its Life |publisher=San Diego History Center |journal=The Journal of San Diego History |date=Winter 1975 |volume=21 |access-date=June 25, 2023 |issue=1 |archive-date=June 26, 2023 |archive-url=https://web.archive.org/web/20230626013218/https://sandiegohistory.org/journal/1975/january/imperial-2/ |url-status=live }}

The Alamo Canal experienced continual problems due to the Colorado's high sediment content and its varying water levels. During low flows, the river often dropped below the level of the canal intake, while high flows silted up the intake, forcing the repeated excavation of new cuts. In early 1905, flooding destroyed the intake gates and water began to flow uncontrolled down the canal towards the Salton Sink. By August, the breach had grown large enough to swallow the entire flow of the river, which began to flood the bottom of the valley. The Southern Pacific Railroad attempted to dam the flow in order to protect their tracks which ran through the valley, but was hampered by repeated flooding. It took seven attempts, more than $3 million, and two years for the railroad, the CDC, and the federal government to permanently block the breach and restore the river's original course – but not before part of the Imperial Valley was flooded under a {{convert|45|mi|km|adj=mid|-long}} lake, today's Salton Sea. The Imperial Valley fiasco demonstrated that further economic development of the region would require a dam to control the Colorado's unpredictable flows.Patten, McCaskie and Unitt, pp. 4–5{{cite web |url=http://www.semp.us/publications/biot_reader.php?BiotID=505 |title=Origin of the Salton Sea |publisher=Suburban Emergency Management Project |archive-url=https://web.archive.org/web/20110718140458/http://www.semp.us/publications/biot_reader.php?BiotID=505 |archive-date=July 18, 2011}}{{sfn|Billington|Jackson|Melosi|2005|pp=141–142}}

Image:HooverDamConstruction.jpg

Image:IMPERIAL_DAM_TAKES_LAST_OF_COLORADO_RIVER_WATER_FOR_THE_UNITED_STATES._IT_DIVERTS_WATER_INTO_ALL-AMERICAN_CANAL...._-_NARA_-_548847.jpg

A large dam on the Colorado River had been envisioned since the 1920s. In 1928, Congress authorized the Reclamation Service (today's U.S. Bureau of Reclamation, or USBR) to build the Boulder Canyon Project, whose key feature would be a dam on the Colorado in Black Canyon {{convert|30|mi|km}} southeast of Las Vegas, Nevada. On September 30, 1935, Hoover Dam was completed, forming Lake Mead, capable of holding more than two years of the Colorado's flow. Lake Mead was, and still is, the largest artificial lake in the U.S. by storage capacity.{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=Boulder%20Canyon%20Project%20-%20Hoover%20Dam |title=Boulder Canyon Project – Hoover Dam |publisher=U.S. Bureau of Reclamation |date=February 1, 2012 |access-date=October 9, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20121011123506/http://www.usbr.gov/projects/Project.jsp?proj_Name=Boulder%20Canyon%20Project%20-%20Hoover%20Dam |archive-date=October 11, 2012}} The construction of Hoover Dam stabilized the lower channel of the Colorado River, stored water for irrigation in times of drought, captured sediment and controlled floods. Hoover was the tallest dam in the world at the time of construction and also had the world's largest hydroelectric power plant.{{cite web |url=http://www.usbr.gov/lc/region/pao/brochures/faq.html |title=The Colorado River and Hoover Dam: Facts and Figures |publisher=U.S. Bureau of Reclamation |date=January 2012 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120514124746/http://www.usbr.gov/lc/region/pao/brochures/faq.html |archive-date=May 14, 2012}}

The Boulder Canyon Project Act also authorized the All-American Canal,{{cite web |url=https://www.archives.gov/milestone-documents/boulder-canyon-project-act |title=Boulder Canyon Project Act (1928) |publisher=National Archives |date=September 21, 2021 |access-date=May 18, 2022 |archive-date=May 18, 2022 |archive-url=https://web.archive.org/web/20220518173242/https://www.archives.gov/milestone-documents/boulder-canyon-project-act |url-status=live }} which was built as a permanent replacement for the Alamo Canal and follows a route entirely within the U.S. on its way to the Imperial Valley. The canal's intake is located at Imperial Dam, {{convert|20|mi|km}} above Yuma, Arizona, which diverts the majority of the Colorado's flow with only a small portion continuing to Mexico. With a capacity of over {{convert|26000|cuft/s|m3/s}}, the All-American Canal is the largest irrigation canal in the world.{{cite web |url=http://earthobservatory.nasa.gov/IOTD/view.php?id=37078 |title=All-American Canal, California–Mexico border |publisher=National Aeronautics and Space Administration |work=Earth Observatory |date=February 23, 2009 |access-date=February 20, 2012 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071906/https://earthobservatory.nasa.gov/images/37078/all-american-canal |url-status=live }} Because the hot, sunny climate lends to a year-round growing season, the Imperial Valley has become one of the most productive farming regions in North America, providing much of the winter produce supply in the U.S.{{Cite encyclopedia |title=Imperial Valley |encyclopedia=Encyclopædia Britannica |year=1995}} The Imperial Irrigation District supplies water to {{convert|520000|acre|ha}} south of the Salton Sea.{{cite web |url=https://www.iid.com/water/about-iid-water |title=About IID Water |publisher=Imperial Irrigation District |date= |access-date=May 18, 2022 |archive-date=May 6, 2022 |archive-url=https://web.archive.org/web/20220506235625/https://www.iid.com/water/about-iid-water |url-status=live }} The Coachella Canal, which branches northward from the All-American Canal, irrigates another {{convert|78000|acre|ha}} in the Coachella Valley.{{cite web |url=https://www.usbr.gov/projects/index.php?id=492 |title=CRBSCP - Coachella Canal Unit - Title I |publisher=U.S. Bureau of Reclamation |date= |access-date=May 18, 2022 |archive-date=May 19, 2022 |archive-url=https://web.archive.org/web/20220519013457/https://www.usbr.gov/projects/index.php?id=492 |url-status=live }}

Parker Dam was initially built as the diversion point for the Colorado River Aqueduct, planned by the Metropolitan Water District of Southern California to supply water to Los Angeles.{{cite web |url=http://www.usbr.gov/projects/Facility.jsp?fac_Name=Parker+Dam&groupName=General |title=Parker Dam |publisher=U.S. Bureau of Reclamation |work=Parker-Davis Project |date=June 29, 2009 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120927195113/http://www.usbr.gov/projects/Facility.jsp?fac_Name=Parker+Dam&groupName=General |archive-date=September 27, 2012}} The construction of the dam was opposed by Arizona, which feared that California would take too much water from the Colorado; at one point, Arizona sent members of its National Guard to stop work on the dam. Ultimately, a compromise was reached, with Arizona dropping its objections in exchange for the USBR constructing the Gila Project, which irrigates {{convert|110000|acre|km2}} on the Arizona side of the river.{{cite web |url=http://www.usbr.gov/projects//ImageServer?imgName=Doc_1305042259136.pdf |title=Gila Project |author=Marie Bell, Tina |publisher=U.S. Bureau of Reclamation |year=1997 |access-date=February 28, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120224024443/http://www.usbr.gov/projects//ImageServer?imgName=Doc_1305042259136.pdf |archive-date=February 24, 2012}} By 1941, the {{convert|241|mi|km|adj=on}} long Colorado River Aqueduct was completed, delivering {{convert|1.2|e6acre.ft|km3}} of water west to Southern California. The aqueduct enabled the continued growth of Los Angeles and its suburbs, and provides water to about 10 million people today.{{cite web |url=http://wsoweb.ladwp.com/Aqueduct/historyoflaa/coloradoriver.htm |title=The Colorado River: A Regional Solution |publisher=Los Angeles Department of Water and Power |access-date=February 28, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120304000641/http://wsoweb.ladwp.com/Aqueduct/historyoflaa/coloradoriver.htm |archive-date=March 4, 2012}} The San Diego Aqueduct, which branches off from the Colorado River Aqueduct in Riverside County, California, opened in stages between 1954 and 1971 and provides water to another 3 million people in the San Diego metro area.{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=San+Diego+Project |title=San Diego Project |publisher=U.S. Bureau of Reclamation |date=January 11, 2012 |access-date=March 16, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120316220454/http://www.usbr.gov/projects/Project.jsp?proj_Name=San+Diego+Project |archive-date=March 16, 2012}}

The Las Vegas Valley of Nevada experienced rapid growth after Hoover Dam, and by 1937 Las Vegas had tapped a pipeline into Lake Mead. Nevada officials, believing that groundwater resources in the southern part of the state were sufficient for future growth, were more concerned with securing a large amount of the dam's power supply than water from the Colorado; thus they settled for the smallest water allocation of all the states in the Colorado River Compact.{{cite web |url=http://www.crwua.org/colorado-river/member-states/nevada |title=Nevada |publisher=Colorado River Water Users Association |access-date=November 2, 2013 |archive-url=https://web.archive.org/web/20131104165703/http://www.crwua.org/colorado-river/member-states/nevada |archive-date=November 4, 2013 |url-status=dead}} In 2018, due to declining water levels in Lake Mead, a second pipeline was completed with a lower intake elevation.

{{main|Colorado River Storage Project}}

In the first half of the 20th century, the Upper Basin states, with the exception of Colorado, had developed very little of their water allocations from the Colorado River Compact. By the 1950s, however, water demand was rapidly increasing in Utah's Wasatch Front (Salt Lake City metro area) and the Rio Grande Valley of New Mexico, which both began exploring ways to divert water from the Colorado Basin.{{sfn|Fradkin|p=191|1996}} The Upper Basin states were concerned that they would not be able to use their full Compact allocations due to increasing water demands in the Lower Basin. The Compact requires the Upper Basin to deliver a minimum annual flow of {{convert|7.5|e6acre.ft}} past Lee's Ferry (measured on a 10-year rolling average). Without additional reservoir storage, the Upper Basin states could not utilize their allocations without impacting water deliveries to the Lower Basin in dry years.

Image:Glen_Canyon_Dam_AZ.jpg

In 1956 Congress authorized the USBR to construct the Colorado River Storage Project (CRSP), which planned several large reservoirs on the upper Colorado, Green, Gunnison and San Juan Rivers.{{cite web |url=https://www.usbr.gov/uc/rm/crsp/index.html |title=Colorado River Storage Project |publisher=U.S. Bureau of Reclamation |date= |access-date=May 18, 2022 |archive-date=November 8, 2019 |archive-url=https://web.archive.org/web/20191108020419/https://www.usbr.gov/uc/rm/crsp/index.html |url-status=live }} The initial blueprints for the CRSP included two dams on the Green River within Echo Park Canyon in Dinosaur National Monument – a move criticized by both the National Park Service and environmental groups such as the Sierra Club. The controversy received nationwide media attention, and the USBR dropped its plans for the Dinosaur dams in exchange for increasing the size of a proposed dam at Glen Canyon.{{cite web |url=http://www.nps.gov/history/history/online_books/colorado/chap9.htm |title=Chapter IX: Dinosaur National Monument |publisher=U.S. National Park Service |work=A Survey of the Recreational Resources of the Colorado River Basin |date=September 6, 2004 |access-date=February 20, 2012 |archive-date=November 3, 2013 |archive-url=https://web.archive.org/web/20131103155731/http://www.nps.gov/history/history/online_books/colorado/chap9.htm |url-status=live }}

The controversy associated with Glen Canyon Dam did not build momentum until construction was well underway. Due to Glen Canyon's remote location, most of the American public did not even know of its existence; the few who did contended that it had much greater scenic value than Echo Park. The environmental movement in the American Southwest has opposed the damming and diversion of the Colorado River system due to negative effects on the ecology and natural beauty of the river and its tributaries. During the construction of Glen Canyon Dam (1956–66), environmental organizations{{which|date=September 2022}} vowed{{clarify|date=September 2022}} to block any further development of the river, and a number of later dam and aqueduct proposals were defeated by citizen opposition. Sierra Club leader David Brower fought the dam both during the construction and for many years afterwards until his death in 2000. Brower believed that he was personally responsible for the failure to prevent Glen Canyon's flooding, calling it his "greatest mistake, greatest sin".{{cite web |url=http://www.drainit.org/campaigns/drought/facts3.cfm |title=Historic Opposition to Glen Canyon Dam |publisher=Colorado Riverkeeper |work=The End of Lake Powell Campaign |date=July 20, 2004 |access-date=November 2, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131228020203/http://www.drainit.org/campaigns/drought/facts3.cfm |archive-date=December 28, 2013}}{{cite web |url=http://archive.audubonmagazine.org/fieldnotes/fieldnotes0607.html |title=Resurrection: Buried Treasure |publisher=National Audubon Society |year=2006 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20130120034155/http://archive.audubonmagazine.org/fieldnotes/fieldnotes0607.html |archive-date=January 20, 2013}}

In addition to Glen Canyon Dam, the CRSP includes the Flaming Gorge Dam on the Green River, the Blue Mesa, Morrow Point and Crystal Dams on the Gunnison River, and the Navajo Dam on the San Juan River. A total of 22 "participating projects" (of which 16 have been constructed) were later authorized in order to develop local water supplies at various locations across the Upper Basin states. These include the Central Utah Project, which delivers {{convert|102000|acre feet|m3}} per year from the Green River basin to the Wasatch Front, and the San Juan–Chama Project, which diverts {{convert|110000|acre feet|m3}} per year from the San Juan River to the Rio Grande Valley. Both are multi-purpose projects serving a variety of agricultural, municipal and industrial uses.{{cite web |url=https://www.doi.gov/cupcao/Overview |title=The Central Utah Project - An Overview |publisher=U.S. Department of the Interior |date= April 3, 2019|access-date=May 18, 2022 |archive-date=May 20, 2022 |archive-url=https://web.archive.org/web/20220520093204/https://www.doi.gov/cupcao/Overview |url-status=live }}{{cite web |url=https://www.usbr.gov/projects/pdf.php?id=186 |title=San Juan-Chama Project |publisher=U.S. Bureau of Reclamation |author=Glaser, Leah S. |date= |access-date=May 18, 2022 |archive-date=March 8, 2022 |archive-url=https://web.archive.org/web/20220308132051/https://www.usbr.gov/projects/pdf.php?id=186 |url-status=live }}

Image:Arizona_cap_canal.jpg, crossing the Sonoran Desert]]

By the middle of the 20th century, planners were concerned that continued growth in water demand would outstrip the available water supply from the Colorado River. After exploring a multitude of potential projects, the USBR published a study in January 1964 known as the Pacific Southwest Water Plan, which proposed diverting water from the northwestern United States into the Colorado River basin.{{cite web |url=https://www.usbr.gov/lc/region/programs/crbstudy/PSWPRptJan64.pdf |title=Pacific Southwest Water Plan |publisher=U.S. Bureau of Reclamation |date=January 1964 |access-date=May 18, 2022 |archive-date=September 4, 2021 |archive-url=https://web.archive.org/web/20210904064353/https://www.usbr.gov/lc/region/programs/crbstudy/PSWPRptJan64.pdf |url-status=live }} Arizona's water allocation was a significant focus of the plan, due to the growing concern that its water supply could be curtailed due to California's seniority of water rights. In addition, the plan would guarantee full water supplies to Nevada, California and Mexico, allowing the Upper Basin states to utilize their full allocations without risking reductions in the Lower Basin. The project would cost an estimated $3.1 billion.{{cite news |url=https://www.nytimes.com/1964/03/15/archives/us-plan-for-dams-on-colorado-under-fire-by-conservationists.html |title=U.S. Plan for Dams on Colorado Under Fire by Conservationists; Interior Department Insists Projects Are Vital to Southwest's Development —National Battle Expected |work=The New York Times |date=March 15, 1964 |access-date=May 18, 2022 |archive-date=May 19, 2022 |archive-url=https://web.archive.org/web/20220519013449/https://www.nytimes.com/1964/03/15/archives/us-plan-for-dams-on-colorado-under-fire-by-conservationists.html |url-status=live }}

The first stage of this plan would divert water from Northern California's Trinity, Klamath and Eel Rivers to Southern California, allowing more Colorado River water to be used, by exchange, in Arizona. A canal system, which ultimately would become the Central Arizona Project (CAP), would be constructed to deliver Arizona's Colorado River allocation to Phoenix and Tucson, both located far away from the Colorado River in the middle of the state. At this point, central Arizona was still entirely dependent on local water supplies, such as the 1911 Theodore Roosevelt Dam,{{cite web |url=http://www.usbr.gov/lc/region/programs/Roosevelt/story.html |title=Reclamation Helps Celebrate Theodore Roosevelt Dam's 100th Year |publisher=U.S. Bureau of Reclamation |date=March 2011 |access-date=March 16, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120925050455/http://www.usbr.gov/lc/region/programs/Roosevelt/story.html |archive-date=September 25, 2012}} and was quickly running out of surplus water.

In order to supply the massive amount of power required to pump Colorado River water to central Arizona, two hydroelectric dams were proposed in the Grand Canyon (Bridge Canyon Dam and Marble Canyon Dam), which while not directly located in Grand Canyon National Park, would greatly impact flows of the Colorado River through the park. With the controversy over Glen Canyon Dam still ongoing, the public pressure against these dams was immense. As a result, the two Grand Canyon dams were omitted from the final CAP authorization in 1968.{{cite web |url=https://grcahistory.org/sites/colorado-river-corridor/bridge-canyon/ |title=Bridge Canyon |publisher=Arizona State University |date= |author=Gerke, Sarah Bohl |access-date=May 18, 2022 |archive-date=February 5, 2021 |archive-url=https://web.archive.org/web/20210205222136/https://grcahistory.org/sites/colorado-river-corridor/bridge-canyon/ |url-status=live }} In addition, the boundaries of Grand Canyon National Park were redrawn to prevent future dam projects in the area. The pumping power was replaced by the building of the coal-fired Navajo Generating Station near Page, Arizona, in 1976.{{cite web |last=Murdock |first=J. Neil |url=http://www.gcrg.org/bqr/10-1/bcd.html |title=Bridge Canyon Dam |publisher=Grand Canyon River Guides |work=from Early History of the Colorado River Storage Project May 1971 U. S. Department of the Interior, Bureau of Reclamation |year=1996 |access-date=July 15, 2013 |archive-url=https://web.archive.org/web/20170525095548/http://www.gcrg.org/bqr/10-1/bcd.html |archive-date=May 25, 2017 |url-status=dead}}{{cite web |url=http://www.nps.gov/history/history/online_books/colorado/chap7.htm |title=Chapter VII: The Grand Canyon |publisher=U.S. National Park Service |work=A Survey of the Recreational Resources of the Colorado River Basin |date=September 6, 2005 |access-date=July 15, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131106001019/http://www.nps.gov/history/history/online_books/colorado/chap7.htm |archive-date=November 6, 2013}} In 2019, the Navajo Generating Station ceased operation.{{cite web |url=https://www.srpnet.com/about/stations/ngs/default.aspx |title=Navajo Generating Station, Project Information |publisher=Salt River Project |access-date=May 18, 2022 |archive-date=May 19, 2022 |archive-url=https://web.archive.org/web/20220519080022/https://www.srpnet.com/about/stations/ngs/default.aspx |url-status=live }}

The CAP was constructed in stages from 1973 to 1993, ultimately extending {{convert|336|mi|km}} from the Colorado River at Parker Dam to Tucson, Arizona. It delivers {{convert|1.4|e6acre.ft|km3}} of water per year, irrigates {{convert|830000|acre|km2}} of farmland and provides municipal water to about 5 million people.{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=Central+Arizona+Project |title=Central Arizona Project |publisher=U.S. Bureau of Reclamation |date=April 18, 2011 |access-date=February 20, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20110613215932/http://www.usbr.gov/projects/Project.jsp?proj_Name=Central+Arizona+Project |archive-date=June 13, 2011}} Due to environmental concerns, most of the facilities proposed in the Pacific Southwest Water Plan were never built (though a smaller version of the Trinity River project was constructed as part of the unrelated Central Valley Project),{{cite web |url=https://www.usbr.gov/mp/ncao/trinity-river.html |title=Trinity River |publisher=U.S. Bureau of Reclamation |date=May 18, 2022 |access-date=May 19, 2022 |archive-date=May 19, 2022 |archive-url=https://web.archive.org/web/20220519013457/https://www.usbr.gov/mp/ncao/trinity-river.html |url-status=live }} leaving Arizona and Nevada vulnerable to future water reductions under the Compact.{{cite web |url=https://sgp.fas.org/crs/misc/R45546.pdf |title=Management of the Colorado River: Water Allocations, Drought, and the Federal Role |publisher=Federation of American Scientists |author=Congressional Research Service |date=August 16, 2021 |access-date=May 18, 2022 |archive-date=May 11, 2022 |archive-url=https://web.archive.org/web/20220511173029/https://sgp.fas.org/crs/misc/R45546.pdf |url-status=live }}

Water rights of Native Americans in the Colorado River basin were largely ignored during the extensive water resources development carried out on the river and its tributaries in the 19th and 20th centuries. The construction of dams has often had negative impacts on tribal peoples, such as the Chemehuevi when their riverside lands were flooded after the completion of Parker Dam in 1938. Ten Native American tribes in the basin now hold or continue to claim water rights to the Colorado River.{{cite web |url=http://www.crwua.org/colorado-river/ten-tribes |title=Ten Tribes Partnership |publisher=Colorado River Water Resources Association |access-date=November 2, 2013 |archive-url=https://web.archive.org/web/20131104165709/http://www.crwua.org/colorado-river/ten-tribes |archive-date=November 4, 2013 |url-status=dead}} The U.S. government has taken some actions to help quantify and develop the water resources of Native American reservations. The first federally funded irrigation project in the U.S. was the construction of an irrigation canal on the Colorado River Indian Reservation in 1867.{{cite web |author=Voggesser, Garrit |url=http://www.usbr.gov/history/ProjectHistories/INDIAN%20PROJECTS%20OVERVIEW.pdf |title=The Indian Projects |publisher=U.S. Bureau of Reclamation |year=2001 |work=Bureau of Reclamation History Program |access-date=April 10, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120224015718/http://www.usbr.gov/history/ProjectHistories/INDIAN%20PROJECTS%20OVERVIEW.pdf |archive-date=February 24, 2012}} Other water projects include the Navajo Indian Irrigation Project, authorized in 1962 for the irrigation of lands in part of the Navajo Nation in north-central New Mexico.{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=Navajo%20Indian%20Irrigation%20Project |title=Navajo Indian Irrigation Project |publisher=U.S. Bureau of Reclamation |date=May 11, 2011 |access-date=April 10, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120925123839/http://www.usbr.gov/projects/Project.jsp?proj_Name=Navajo%20Indian%20Irrigation%20Project |archive-date=September 25, 2012}} The Navajo continue to seek expansion of their water rights because of difficulties with the water supply on their reservation; about 40 percent of its inhabitants must haul water by truck many miles to their homes. In the 21st century, they have filed legal claims against the governments of Arizona, New Mexico and Utah for increased water rights. Some of these claims have been successful for the Navajo, such as a 2004 settlement in which they received a {{convert|326000|acre foot|ML|adj=on}} allotment from New Mexico.{{cite news |author=Jenkins, Matt |title=In Navajoland, a Contentious Water Deal Divides the Tribe |work=High Country News |location=Paonia, Colorado |url=http://www.hcn.org/issues/43.3/in-navajoland-a-contentious-water-deal-divides-the-tribe |date=February 21, 2011 |access-date=July 1, 2013 |archive-date=January 25, 2021 |archive-url=https://web.archive.org/web/20210125015648/https://www.hcn.org/issues/43.3/in-navajoland-a-contentious-water-deal-divides-the-tribe |url-status=live }}

{{See also|Southwestern North American megadrought}}

{{rquote|right|[The Colorado is] a 'deficit' river, as if the river were somehow at fault for its overuse.|Marc Reisner, in Cadillac Desert{{sfn|Reisner|p=121|1993}}}}

When the Colorado River Compact was drafted in the 1920s, it was based on barely 30 years of streamflow records that suggested an average annual flow of {{convert|17.5|e6acre.ft|km3}} past Lee's Ferry.{{sfn|Reisner|p=126|1993}} Modern studies of tree rings revealed that those three decades were probably the wettest in the past 500 to 1,200 years and that the natural long-term annual flow past Lee's Ferry is probably closer to {{convert|13.5|e6acre.ft|km3}},{{cite journal |author1=Stahle, David W. |author2=Fye, Falko K. |author3=Therrell, Matthew D. |title=Interannual to Decadal Climate and Streamflow Variability Estimated from Tree Rings |doi=10.1016/S1571-0866(03)01023-6 |journal=Developments in Quaternary Science |date=2003 |volume=1 |pages=491–504 |series=Developments in Quaternary Sciences |bibcode=2003DevQS...1..491S |isbn=9780444514707}}{{refn|The discrepancy between the natural flow at Lee's Ferry (13.5 million acre-feet or 16.65 km³) and the gauged flow between 1922 and 2020 (10.58 million acre-feet or 13.05 km³) is mostly due to water diversions above Lee's Ferry and evaporation from reservoirs, especially Lake Powell.{{cite web |url=http://www.usbr.gov/uc/library/envdocs/reports/crs/pdfs/cul2006-2010prov.pdf |title=Upper Colorado River Basin Consumptive Uses and Losses Report 2006-2010 |publisher=U.S. Bureau of Reclamation |date=August 2012 |access-date=June 29, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20121010175929/http://www.usbr.gov/uc/library/envdocs/reports/crs/pdfs/cul2006-2010prov.pdf |archive-date=October 10, 2012}}|group=n}} with a natural flow at the mouth around {{convert|16.3|e6acre.ft|km3}}.{{cite web |author=Nowak, Kenneth C. |title=Stochastic Streamflow Simulation at Interdecadal Time Scales and Implications to Water Resources Management in the Colorado River Basin |publisher=University of Colorado |work=Center for Advanced Decision Support for Water and Environmental Systems |url=http://cadswes.colorado.edu/sites/default/files/PDF/Theses-PhD/Nowak-PhD.pdf |date=April 2, 2012 |page=114 |access-date=July 11, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20140501143218/http://cadswes.colorado.edu/sites/default/files/PDF/Theses-PhD/Nowak-PhD.pdf |archive-date=May 1, 2014}} This has resulted in more water being allocated to river users than actually exists in the Colorado.{{cite journal |author=Gelt, Joe |url=http://worldcat.org/arcviewer/2/WCA/2009/12/14/H1260824596711/viewer/file2.html |title=Sharing Colorado River Water: History, Public Policy and the Colorado River Compact |journal=Arroyo |publisher=University of Arizona, Water Resources Research Center |volume=10 |date=August 1997 |issue=1 |access-date=February 25, 2014 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071625/http://worldcat.org/arcviewer/2/WCA/2009/12/14/H1260824596711/viewer/file2.html |url-status=live }} Droughts have exacerbated the issue of water over-allocation.{{cite web |url=http://www.riverguides.org/Confluence/27/27DroughtWeisheit.pdf |title=Persistent Drought in the Colorado River Basin |author=Weisheit, John |work=Confluence |volume=27 |access-date=May 15, 2012 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071530/http://www.riverguides.org/Confluence/27/27DroughtWeisheit.pdf |url-status=live }}{{cite web |url=http://www.fs.fed.us/psw/cirmount/policy/pdf/Paleo_Arc.pdf |title=Paleoenvironmental Archives as a source of Climate Information for Natural Resource Management: An Example from Tree Rings and Colorado Water Management |publisher=U.S. Forest Service |author1=Woodhouse, Connie |author-link1=Connie Woodhouse |author2=Webb, Robert S. |author3=Lukas, Jeff |access-date=May 19, 2012 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071545/https://www.fs.fed.us/psw/cirmount/policy/pdf/Paleo_Arc.pdf |url-status=live }}

File:Lake Mead October 2010.jpg

The most severe drought on record, the southwestern North American megadrought, began in the early 21st century, in which the river basin has produced above-average runoff in only five years between 2000 and 2021.{{cite web |url=https://www.usbr.gov/uc/water/crsp/cs/gcd.html |title=Glen Canyon Dam Water Operations |publisher=U.S. Bureau of Reclamation |year=2022 |access-date=May 18, 2022 |archive-date=September 28, 2020 |archive-url=https://web.archive.org/web/20200928134447/https://www.usbr.gov/uc/water/crsp/cs/gcd.html |url-status=live }} The region is experiencing a warming trend, which is accompanied by earlier snowmelt, lower precipitation and greater evapotranspiration. A 2004 study showed that a 1–6 percent decrease of precipitation would lead to runoff declining by as much as 18 percent by 2050.{{cite web |url=http://wwa.colorado.edu/climate_change/docs/Hoerling%20v2%20WWA%20Front%20Range%20Water.pdf |author1=Hoerling, Martin |author2=Eischeid, Jon |title=Outlook for Water in the West at 2050 |publisher=Western Water Assessment |work=NOAA Earth System Research Laboratory, Climate Change Workshop |date=November 17, 2006 |archive-url=https://web.archive.org/web/20100609222623/http://wwa.colorado.edu/climate_change/docs/Hoerling%20v2%20WWA%20Front%20Range%20Water.pdf |archive-date=June 9, 2010}}

Since 2000, reservoir levels have fluctuated greatly from year to year, but have experienced a steady long-term decline.{{cite web |url=http://www.crwcd.org/media/uploads/CO_River_Uncertain_Future.pdf |title=The Colorado River's Uncertain Future: How Climate Change May Affect Future Planning Decisions on the Colorado River |publisher=Colorado River Water Conservation District |author=Kuhn, Eric |work=CRWCD 49th Annual Convention |date=January 26, 2007 |access-date=February 21, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20111201024238/http://www.crwcd.org/media/uploads/CO_River_Uncertain_Future.pdf |archive-date=December 1, 2011}} The particularly dry spell between 2000 and 2004 brought Lake Powell to just a third of capacity in 2005, the lowest level on record since initial filling in 1969.{{cite web |author=Vernieu-Presenter, William S. |url=http://www.gcrg.org/docs/gtslib/vernieu-05.pdf |title=Current Status and Trends of Lake Powell and Glen Canyon Dam Release Water Quality |publisher=Grand Canyon River Guides |work=U.S. Geological Survey, Grand Canyon Monitoring and Research Center |year=2005 |access-date=May 15, 2012 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071038/http://www.gcrg.org/docs/gtslib/vernieu-05.pdf |url-status=dead}} In late 2010, Lake Mead was approaching the "drought trigger" elevation of {{convert|1075|ft|m}}, at which water supplies to Arizona and Nevada would be reduced in accordance with the Colorado River Compact.{{cite news |url=http://www.azcentral.com/arizonarepublic/news/articles/2010/10/19/20101019lake-mead-water-level-new-historic-low.html |author=McKinnon, Shaun |title=Lake Mead Sinks to a New Historic Low: If Lake Falls 8 More Feet, Thirsty Ariz. Could Experience Water Restrictions |newspaper=The Arizona Republic |location=Phoenix, Arizona |date=October 19, 2010 |access-date=September 6, 2012 |archive-date=January 17, 2013 |archive-url=https://archive.today/20130117144207/http://www.azcentral.com/arizonarepublic/news/articles/2010/10/19/20101019lake-mead-water-level-new-historic-low.html?nclick_check=1 |url-status=live }} Due to Arizona and Nevada's water rights being junior to California's, their allocations can legally be cut to zero before any reductions are made on the California side.{{cite news |author=Hiltzik, Michael |title=Water war bubbling up between California and Arizona |url=http://www.latimes.com/business/hiltzik/la-fi-hiltzik-20140620-column.html#page=1 |work=Los Angeles Times |date=2014-06-20 |access-date=2014-09-23 |archive-date=June 24, 2021 |archive-url=https://web.archive.org/web/20210624134813/https://www.latimes.com/business/hiltzik/la-fi-hiltzik-20140620-column.html#page=1 |url-status=live }}

A wet winter in 2011 temporarily raised lake levels,{{cite news |url=http://www.azcentral.com/news/articles/2011/04/18/20110418lake-mead-replenished-by-snowfall.html |author=McKinnon, Shaun |title=Lake Mead Replenished by Snowfall |work=azcentral.com |date=April 19, 2011 |access-date=February 21, 2012 |archive-date=January 17, 2013 |archive-url=https://archive.today/20130117112702/http://www.azcentral.com/news/articles/2011/04/18/20110418lake-mead-replenished-by-snowfall.html?nclick_check=1 |url-status=live }}{{cite news |url=http://www.lasvegassun.com/news/2011/aug/17/lake-meads-water-level-rises/ |title=Lake Mead's water level rises 30 feet after wet winter |author=Shine, Conor |newspaper=Las Vegas Sun |date=August 17, 2011 |access-date=February 21, 2012 |archive-date=July 25, 2016 |archive-url=https://web.archive.org/web/20160725022127/http://lasvegassun.com/news/2011/aug/17/lake-meads-water-level-rises/ |url-status=live }} but dry conditions returned in the next two years.{{cite news |author=Perry, Tony |title=As Colorado River drought worsens, water officials parley |work=Los Angeles Times |date=May 27, 2013 |url=https://www.latimes.com/local/lanow/la-xpm-2013-may-27-la-me-ln-as-drought-worsens-along-colorado-river-water-officials-discuss-moving-forward-20130527-story.html |access-date=July 9, 2013 |archive-date=July 24, 2013 |archive-url=https://web.archive.org/web/20130724233655/http://articles.latimes.com/2013/may/27/local/la-me-ln-as-drought-worsens-along-colorado-river-water-officials-discuss-moving-forward-20130527 |url-status=live }} In 2014, the Bureau of Reclamation cut releases from Lake Powell by 10 percent —the first such reduction since the 1960s, when Lake Powell was being filled for the first time.{{cite news |author=Johnson, Terrell |title=Dwindling Colorado River Forces First-Ever Cuts in Lake Powell Water Releases |url=http://www.weather.com/news/science/environment/drought-lake-powell-lake-mead-climate-change-20130818 |work=The Weather Channel |date=August 20, 2013 |access-date=January 16, 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140113020305/http://www.weather.com/news/science/environment/drought-lake-powell-lake-mead-climate-change-20130818 |archive-date=January 13, 2014}} This resulted in Lake Mead dropping to its lowest recorded level since 1937, when it was first being filled.{{cite news |author=Brean, Henry |title=Lake Mead sinks to a record low |work=Las Vegas Review-Journal |url=http://www.reviewjournal.com/news/nevada/lake-mead-sinks-record-low |date=2014-07-11 |access-date=2014-10-02 |archive-date=January 3, 2015 |archive-url=https://web.archive.org/web/20150103032005/http://www.reviewjournal.com/news/nevada/lake-mead-sinks-record-low |url-status=live }}

Water year 2018 had a much lower-than-average snowpack.{{cite news |url=http://knau.org/post/colorado-river-water-shortage-could-be-declared-2020 |title=Colorado River Water Shortage Could be Declared in 2020 |author=Sevigny, Melissa |work=KNAU |date=2018-05-17 |access-date=2018-05-24 |archive-date=August 18, 2021 |archive-url=https://web.archive.org/web/20210818042930/https://www.knau.org/post/colorado-river-water-shortage-could-be-declared-2020 |url-status=live }}{{cite news |url=https://www.ecowatch.com/colorado-river-water-shortage-2569729456.html |title=Unprecedented Colorado River Water Shortage Could Be Declared in 2020 |author=Chow, Lorraine |work=EcoWatch |date=2018-05-17 |access-date=2018-05-24 |archive-date=May 24, 2020 |archive-url=https://web.archive.org/web/20200524233910/https://www.ecowatch.com/colorado-river-water-shortage-2569729456.html |url-status=live }} In July 2021, after two more extremely dry winters, Lake Powell fell below the previous low set in 2005. In response, the Bureau of Reclamation began releasing water from upstream reservoirs in order to keep Powell above the minimum level for hydropower generation.{{cite news |url=https://www.sltrib.com/news/environment/2021/07/22/lake-powell-level-about/ |title=Lake Powell level about to hit a historic low as West's water crisis deepens |author=Brian Maffly |work=Salt Lake Tribune |date=2021-07-22 |access-date=July 31, 2021 |archive-date=August 18, 2021 |archive-url=https://web.archive.org/web/20210818095410/https://www.sltrib.com/news/environment/2021/07/22/lake-powell-level-about/ |url-status=live }}{{cite news |url=https://www.cpr.org/2021/07/24/lake-powell-lowest-water-level-on-record-heres-what-that-means-for-colorado-and-states-that-rely-on-it/ |title=Lake Powell Just Hit Its Lowest Level On Record. Here's What That Means For Colorado And States That Rely On It |work=Colorado Public Radio |author=Michael Elizabeth Sakas |date=2021-07-24 |access-date=July 31, 2021 |archive-date=August 15, 2021 |archive-url=https://web.archive.org/web/20210815042611/https://www.cpr.org/2021/07/24/lake-powell-lowest-water-level-on-record-heres-what-that-means-for-colorado-and-states-that-rely-on-it/ |url-status=live }} Lake Mead fell below the {{convert|1075|ft|m|adj=on}} level expected to trigger federally mandated cuts to Arizona and Nevada's water supplies for the first time in history, and is expected to continue declining into 2022.{{cite news |url=https://www.theguardian.com/us-news/2021/jul/13/hoover-dam-lake-mead-severe-drought-us-west |title=Severe drought threatens Hoover dam reservoir – and water for US west |work=The Guardian |author=Oliver Milman |date=2021-07-13 |access-date=July 31, 2021 |archive-date=August 25, 2021 |archive-url=https://web.archive.org/web/20210825214104/https://www.theguardian.com/us-news/2021/jul/13/hoover-dam-lake-mead-severe-drought-us-west |url-status=live }}

On August 16, 2021, the Bureau of Reclamation released the Colorado River Basin August 2021 24-Month Study, and for the first time declared a shortage and that because of "ongoing historic drought and low runoff conditions in the Colorado River Basin, downstream releases from Glen Canyon Dam and Hoover Dam will be reduced in 2022 due to declining reservoir levels."{{cite web |url=https://www.usbr.gov/newsroom/#/news-release/3950 |title=Reclamation announces 2022 operating conditions for Lake Powell and Lake Mead |date=Aug 16, 2021 |website=usbr.gov |publisher=United States Bureau of Reclamation |access-date=2021-08-19 |archive-date=August 17, 2021 |archive-url=https://archive.today/20210817005912/https://www.usbr.gov/newsroom/%23/news-release/3950 |url-status=live }}

The Lower Basin reductions will reduce the annual apportionments – Arizona's by 18 percent, Nevada's by 7 percent, and Mexico's by 5 percent.{{cite web |url=https://www.msn.com/en-us/news/politics/overnight-energy-federal-government-announces-first-ever-water-shortage-in-lake-mead-colorado-river-biden-administration-releases-guidance-limiting-international-financing-for-fossil-fuels-biden-administration-to-review-federal-coal-leasing/ar-AANocbi |title=OVERNIGHT ENERGY: Federal government announces first-ever water shortage in Lake Mead, Colorado River |Biden administration releases guidance limiting international financing for fossil fuels |Biden administration to review federal coal leasing |last=Budryk |first=Zack |date=Aug 17, 2021 |website=msn.com |publisher=MSN |access-date=2021-08-19 |archive-date=August 28, 2021 |archive-url=https://archive.today/20210828075952/https://www.msn.com/en-us/news/politics/overnight-energy-federal-government-announces-first-ever-water-shortage-in-lake-mead-colorado-river-biden-administration-releases-guidance-limiting-international-financing-for-fossil-fuels-biden-administration-to-review-federal-coal-leasing/ar-AANocbi |url-status=live }}

On June 14, 2022, Bureau of Reclamation Commissioner Camille Touton told the Senate Committee on Energy and Natural resources that additional cuts of 2–4 million acre-feet were required to stabilize reservoir levels in 2023. Touton warned that if states were unable to negotiate the requisite cuts the Interior department may use its legal authority to cut releases.{{cite web |title=Full Committee Hearing To Examine Short And Long Term Solutions To Extreme Drought In The Western U.S. |url=https://www.energy.senate.gov/hearings/2022/6/full-committee-hearing-to-examine-short-and-long-term-solutions-to-extreme-drought-in-the-western-u-s |website=Senate Committee on Energy and Natural resources |date=June 14, 2022 |publisher=United States Senate |access-date=16 December 2022 |archive-date=December 16, 2022 |archive-url=https://web.archive.org/web/20221216053044/https://www.energy.senate.gov/hearings/2022/6/full-committee-hearing-to-examine-short-and-long-term-solutions-to-extreme-drought-in-the-western-u-s |url-status=live }} When the states were unable to come to an agreement about how to share the proposed cuts, Reclamation began the legal steps to unilaterally reduce releases from Hoover and Glen Canyon Dams in 2023.{{cite news |last1=Trotta |first1=Daniel |title=U.S. warns western states it may impose Colorado River water cuts |url=https://www.reuters.com/world/us/us-warns-western-states-it-may-impose-colorado-river-water-cuts-2022-10-28/ |access-date=16 December 2022 |work=Reuters |date=28 October 2022 |archive-date=December 16, 2022 |archive-url=https://web.archive.org/web/20221216053042/https://www.reuters.com/world/us/us-warns-western-states-it-may-impose-colorado-river-water-cuts-2022-10-28/ |url-status=live }} As of December 2022 the lower basin states of Nevada, Arizona, and California had not agreed on how to reduce water use by the approximately 30% required to keep levels in lakes Mead and Powell from crashing.{{cite news |last1=Lochhead |first1=Colton |title=Las Vegas water boss urges states to take action to keep lakes from crashing |url=https://www.reviewjournal.com/news/politics-and-government/las-vegas-water-boss-urges-states-to-take-action-to-keep-lakes-from-crashing-2695166/ |access-date=16 December 2022 |agency=Las Vegas Review Journal |date=15 December 2022 |archive-date=December 16, 2022 |archive-url=https://web.archive.org/web/20221216053042/https://www.reviewjournal.com/news/politics-and-government/las-vegas-water-boss-urges-states-to-take-action-to-keep-lakes-from-crashing-2695166/ |url-status=live }} The Bureau of Reclamation had projected that water levels at Lake Powell could fall low enough that by July 2023 Glen Canyon Dam would no longer be able to generate any hydropower, a scenario referred to as "dead pool".{{cite news |last1=Partlow |first1=Joshua |title=Officials fear 'complete doomsday scenario' for drought-stricken Colorado River |url=https://www.washingtonpost.com/climate-environment/2022/12/01/drought-colorado-river-lake-powell/ |access-date=16 December 2022 |agency=The Washington Post |date=1 December 2022 |archive-url=https://web.archive.org/web/20221209152836/https://www.washingtonpost.com/climate-environment/2022/12/01/drought-colorado-river-lake-powell/ |archive-date=December 9, 2022}} Arizona proposed a plan that severely cut allocations to California, and California responded with a plan that severely cut allocations to Arizona, failing to reach consensus. In April 2023, the federal government proposed cutting allocations to Nevada, Arizona, and California evenly which would cut deliveries by as much as one-quarter to each state, rather than according to senior water rights.{{cite news |url=https://www.nytimes.com/2023/04/11/climate/colorado-river-water-cuts-drought.html |title=White House Proposes Evenly Cutting Water Allotments From Colorado River |newspaper=The New York Times |author=Christopher Flavelle |date=April 11, 2023 |access-date=April 13, 2023 |archive-date=April 13, 2023 |archive-url=https://web.archive.org/web/20230413192525/https://www.nytimes.com/2023/04/11/climate/colorado-river-water-cuts-drought.html |url-status=live }}

In May 2023, the states finally reached a temporary agreement to prevent a "dead pool", reducing allocations by 3 million acre-feet over three years (until the end of 2026). 700,000 acre-feet were to be negotiated later among California, Arizona, and Nevada.{{cite news |url=https://www.sltrib.com/news/nation-world/2023/05/22/do-or-die-talks-reach-deal-keep/ |title=Do-or-die talks reach deal to keep Colorado River from going dry, for now |author=Christopher Flavelle |newspaper=The New York Times |date=May 22, 2023 |access-date=June 10, 2023 |archive-date=June 10, 2023 |archive-url=https://web.archive.org/web/20230610013841/https://www.sltrib.com/news/nation-world/2023/05/22/do-or-die-talks-reach-deal-keep/ |url-status=live }} The cuts were less than the federal government had demanded, and so further cuts will be needed after 2026. Fewer cuts were needed in the short term because the Colorado River Basin experienced an unusually rainy and snowy weather in early 2023.{{cite news |url=https://www.latimes.com/environment/story/2023-04-08/colorado-river-snowpack-california |title=Drought-ravaged Colorado River gets relief from snow. But long-term water crisis remains |author=Ian James |newspaper=The Los Angeles Times |date=8 April 2023 |access-date=April 13, 2023 |archive-date=April 13, 2023 |archive-url=https://web.archive.org/web/20230413210949/https://www.latimes.com/environment/story/2023-04-08/colorado-river-snowpack-california |url-status=live }}

The agreement also became easier to negotiate because many cuts are being offset by one-time federal funding.{{Cite web |url=https://www.reuters.com/world/us/us-states-reach-colorado-river-water-conservation-deal-interior-dept-2023-05-22/ |title=Western states reach 'historic' deal to help save Colorado River |website=Reuters |access-date=June 10, 2023 |archive-date=June 10, 2023 |archive-url=https://web.archive.org/web/20230610013840/https://www.reuters.com/world/us/us-states-reach-colorado-river-water-conservation-deal-interior-dept-2023-05-22/ |url-status=live }} Billions of dollars in funding for programs in the Colorado River Basin to recycle water, increase efficiency, and competitive grants to pay water rights holders not to use water from the river are being provided by the Infrastructure Investment and Jobs Act and Inflation Reduction Act, and other programs funded through the United States Environmental Protection Agency and United States Department of the Interior. These are projected to reduce demand by hundreds of thousands of acre-feet per year.{{Cite web |url=https://bidenwhitehouse.archives.gov/briefing-room/statements-releases/2023/04/06/fact-sheet-biden-harris-administration-announces-new-investments-to-protect-the-colorado-river-system/ |title=FACT SHEET: Biden-Harris Administration Announces New Investments to Protect the Colorado River System |first=The White |last=House |date=April 6, 2023 |website=The White House |access-date=April 13, 2023 |archive-date=April 13, 2023 |archive-url=https://web.archive.org/web/20230413231804/https://www.whitehouse.gov/briefing-room/statements-releases/2023/04/06/fact-sheet-biden-harris-administration-announces-new-investments-to-protect-the-colorado-river-system/ |url-status=live }}

{{further|List of flora of the Lower Colorado River Valley}}

Image:Topockgorge.jpg]]

The Colorado River and its tributaries often nourish extensive corridors of riparian growth as they traverse the arid desert regions of the watershed. Although riparian zones represent a relatively small proportion of the basin and have been affected by engineering projects and river diversion in many places, they have the greatest biodiversity of any habitat in the basin.{{cite web |url=http://cpluhna.nau.edu/Biota/riparian_communities.htm |title=Biotic Communities of the Colorado Plateau: Riparian Areas |publisher=Northern Arizona University |work=Land Use History of North America |access-date=February 23, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120213105737/http://www.cpluhna.nau.edu/Biota/riparian_communities.htm |archive-date=February 13, 2012}} The most prominent riparian zones along the river occur along the lower Colorado below Davis Dam,{{cite web |title=The Ecology of the Lower Colorado River from Davis Dam to the Mexico-United States International Boundary: A Community Profile |url=http://www.nwrc.usgs.gov/techrpt/85-7-19.pdf |publisher=U.S. Fish and Wildlife Service |date=September 1988 |first1=Robert D. |last1=Ohmart |first2=Bertin W. |last2=Anderson |first3=William C. |last3=Hunter |pages=9, 12, 14 |access-date=June 29, 2013 |archive-date=June 3, 2013 |archive-url=https://web.archive.org/web/20130603002626/http://www.nwrc.usgs.gov/techrpt/85-7-19.pdf |url-status=live }} especially in the Colorado River Delta, where riparian areas support 358 species of birds despite the reduction in freshwater flow and invasive plants such as tamarisk (salt cedar).{{cite journal |url=http://www.lvwash.org/assets/pdf/resources_ecoresearch_birds_journal.pdf |title=Restoring Habitat for Riparian Birds in the Lower Colorado River Watershed: An Example from the Las Vegas Wash, Nevada |journal=Journal of Arid Environments |date=2011 |pages=1182–1190 |doi=10.1016/j.jaridenv.2011.06.017 |author1=Shanahan, S.A. |author2=Nelson, S.M. |author3=Van Dooremolen, D.M. |author4=Eckberg, J.R. |volume=75 |issue=11 |bibcode=2011JArEn..75.1182S |access-date=February 24, 2012 |archive-date=January 20, 2013 |archive-url=https://web.archive.org/web/20130120034136/http://www.lvwash.org/assets/pdf/resources_ecoresearch_birds_journal.pdf |url-status=dead}} Reduction of the delta's size has also threatened animals such as jaguars and the vaquita porpoise, which is endemic to the gulf.{{cite web |author=Alles, David L. |url=http://fire.biol.wwu.edu/trent/alles/TheDelta.pdf |title=The Delta of the Colorado River |publisher=Western Washington University |date=August 8, 2007 |access-date=February 23, 2012 |archive-date=June 20, 2012 |archive-url=https://web.archive.org/web/20120620100453/http://fire.biol.wwu.edu/trent/alles/TheDelta.pdf |url-status=live }} Human development of the Colorado River has also helped to create new riparian zones by smoothing the river's seasonal flow, notably through the Grand Canyon.{{cite web |author1=Schell, Robert |author2=King, Morgan E. |url=https://www.geology.ucdavis.edu/~shlemonc/html/trips/Grand%20Canyon%20web/html/flogs/PDFs/structuralvegetation.pdf |archive-url=https://wayback.archive-it.org/all/20111206211357/https://www.geology.ucdavis.edu/~shlemonc/html/trips/Grand%20Canyon%20web/html/flogs/PDFs/structuralvegetation.pdf |url-status=dead |archive-date=December 6, 2011 |title=Riparian Vegetation: Structural Diversity Benefits Birds |publisher=University of California, Davis |access-date=February 23, 2012}}

More than 1,600 species of plants grow in the Colorado River watershed, ranging from the creosote bush, saguaro cactus, and Joshua trees of the Sonoran and Mojave Deserts to the forests of the Rocky Mountains and other uplands, composed mainly of ponderosa pine, subalpine fir, Douglas-fir and Engelmann spruce.{{sfn|Benke|Cushing|p=488|2005}} Before logging in the 19th century, forests were abundant in high elevations as far south as the Mexico–U.S. border, and runoff from these areas nourished abundant grassland communities in river valleys. Some arid regions of the watershed, such as the upper Green River valley in Wyoming, Canyonlands National Park in Utah and the San Pedro River valley in Arizona and Sonora, supported extensive reaches of grassland roamed by large mammals such as buffalo and antelope as late as the 1860s. Near Tucson, Arizona, "where now there is only powder-dry desert, the grass once reached as high as the head of a man on horse back".{{cite web |url=http://www.nps.gov/history/history/online_books/colorado/chap3a.htm |title=Status of Plant and Animal Life |publisher=U.S. National Park Service |work=A Survey of the Recreational Resources of the Colorado River Basin |date=September 6, 2004 |access-date=February 23, 2012 |archive-date=May 1, 2014 |archive-url=https://web.archive.org/web/20140501135622/http://www.nps.gov/history/history/online_books/colorado/chap3a.htm |url-status=live }}

Rivers and streams in the Colorado basin were once home to 49 species of native fish, of which 42 were endemic. Engineering projects and river regulation have led to the extinction of four species and severe declines in the populations of 40 species.{{sfn|Benke|Cushing|p=493|2005}} Bonytail chub, razorback sucker, Colorado pikeminnow, and humpback chub are among those considered the most at risk; all are unique to the Colorado River system and well adapted to the river's natural silty conditions and flow variations. Clear, cold water released by dams has significantly changed characteristics of habitat for these and other Colorado River basin fishes.{{cite web |url=http://www.feow.org/ecoregion_details.php?eco=130 |title=130: Colorado |publisher=Freshwater Ecoregions of the World |date=January 11, 2012 |access-date=February 23, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120618091541/http://www.feow.org/ecoregion_details.php?eco=130 |archive-date=June 18, 2012}} A further 40 species that occur in the river today, notably the brown trout, were introduced during the 19th and 20th centuries, mainly for sport fishing.{{cite web |url=http://www.defenders.org/wildlife_and_habitat/wildlife/colorado_river_basin_fish.php |title=Colorado River Basin Fish |publisher=Defenders of Wildlife |archive-url=https://web.archive.org/web/20110406174223/http://www.defenders.org/wildlife_and_habitat/wildlife/colorado_river_basin_fish.php |archive-date=April 6, 2011}}

File:20230522 Colorado River water usage - livestock crops etc.svg

File:Horseshoebend smt.jpg

Historically, the Colorado transported from {{convert|85|to|100|e6ST|t}} of sediment or silt to the Gulf of California each year – second only to the Mississippi among North American rivers.{{sfn|Gupta|p=200|2007}} This sediment nourished wetlands and riparian areas along the river's lower course, particularly in its {{convert|3000|mi2|km2|adj=on}} delta, once the largest desert estuary on the continent.{{cite web |url=https://www.npr.org/sections/pictureshow/2011/07/15/137821595/why-the-colorado-river-stopped-flowing?singlePage=false |title=Why The Colorado River Stopped Flowing |publisher=National Public Radio |date=July 14, 2011 |access-date=February 20, 2012 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819072827/https://www.npr.org/sections/pictureshow/2011/07/15/137821595/why-the-colorado-river-stopped-flowing?singlePage=false |url-status=live }} Currently, the majority of sediments carried by the Colorado River are deposited at the upper end of Lake Powell, and most of the remainder ends up in Lake Mead. Various estimates place the time it would take for Powell to completely fill with silt at 300 to 700 years. Dams trapping sediment not only pose damage to river habitat but also threaten future operations of the Colorado River reservoir system.{{cite web |author=Weisheit, John |url=http://www.riverguides.org/Confluence/27/27SedimentWeisheit.pdf |title=A Colorado River Sediment Inventory |publisher=Colorado Plateau River Guides |access-date=February 20, 2012 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071316/http://www.riverguides.org/Confluence/27/27SedimentWeisheit.pdf |url-status=live }}

Reduction in flow caused by dams, diversions, water for thermoelectric power stations,{{cite web |title=Colorado River Basin |url=http://www.coriverbasin.org/ |publisher=coriverbasin.org |access-date=May 20, 2013 |archive-date=May 29, 2013 |archive-url=https://web.archive.org/web/20130529184259/http://coriverbasin.org/ |url-status=live }} and evaporation losses from reservoirs – the latter of which consumes more than 15 percent of the river's natural runoff{{cite web |url=http://www.usbr.gov/uc/library/envdocs/reports/crs/pdfs/crs962000.pdf |title=Colorado River System Consumptive Uses and Losses Report 1996–2000 |publisher=U.S. Bureau of Reclamation |archive-url=https://web.archive.org/web/20090119165100/http://www.usbr.gov/uc/library/envdocs/reports/crs/pdfs/crs962000.pdf |archive-date=January 19, 2009 |date=December 2004}}—has had severe ecological consequences in the Colorado River Delta and the Gulf of California. Historically, the delta with its large freshwater outflow and extensive salt marshes provided an important breeding ground for aquatic species in the Gulf. Today's desiccated delta, at only a fraction of its former size, no longer provides suitable habitat, and populations of fish, shrimp and sea mammals in the gulf have seen a dramatic decline.{{cite web |author=Davis, Eric F. |url=https://www.msu.edu/~davise16/pdf/The%20Colorado%20River%20Controversy.pdf?q=colorado-river |title=The Colorado River Controversy |publisher=Michigan State University |format=PDF |archive-url=https://web.archive.org/web/20041001045622/http://www.msu.edu/~davise16/pdf/The%20Colorado%20River%20Controversy.pdf |archive-date=October 1, 2004}} Since 1963, the only times when the Colorado River has reached the ocean have been during El Niño events in the 1980s and 1990s.{{cite magazine |author=Sandra Postel |url=http://newswatch.nationalgeographic.com/2013/04/05/landmark-cooperation-brings-the-colorado-river-home/ |archive-url=https://web.archive.org/web/20130407110811/http://newswatch.nationalgeographic.com/2013/04/05/landmark-cooperation-brings-the-colorado-river-home/ |url-status=dead |archive-date=April 7, 2013 |title=Landmark Cooperation Brings the Colorado River Home |magazine=National Geographic |date=April 5, 2013 |access-date=May 19, 2014}}

Reduced flows have led to increases in the concentration of certain substances in the lower river that have impacted water quality. Salinity is one of the major issues and also leads to the corrosion of pipelines in agricultural and urban areas. The lower Colorado's salt content was about 50 parts per million (ppm) in its natural state, but by the 1960s, it had increased to well over 2000 ppm.{{cite web |author1=Oyarzabal-Tamargo, Francisco |author2=Young, Robert A. |url=http://ageconsearch.umn.edu/bitstream/32180/1/01010007.pdf |title=The Colorado River Salinity Problem: Direct Economic Damages in Mexico |publisher=University of Minnesota |date=June 1977 |access-date=February 20, 2012 |archive-date=May 14, 2013 |archive-url=https://web.archive.org/web/20130514074643/http://ageconsearch.umn.edu/bitstream/32180/1/01010007.pdf |url-status=live }} By the early 1970s, there was also serious concern about salinity caused by salts leached from local soils by irrigation drainage water, which were estimated to add {{convert|10|e6ST|t}} of excess salt to the river per year. The Colorado River Basin Salinity Control Act was passed in 1974, mandating conservation practices including the reduction of saline drainage. The program reduced the annual load by about {{convert|1.2|e6ST|t}}, but salinity remains an ongoing issue.{{cite web |url=http://www.socalsalinity.org/pdfs/CRBSCPBriefingDocument.pdf |title=Colorado River Basin Salinity Control Program |publisher=Southern California Salinity Coalition |access-date=June 29, 2013 |archive-date=March 4, 2016 |archive-url=https://web.archive.org/web/20160304123501/http://www.socalsalinity.org/pdfs/CRBSCPBriefingDocument.pdf |url-status=dead}} In 1997, the USBR estimated that saline irrigation water caused crop damages exceeding $500 million in the U.S. and $100 million in Mexico. Further efforts have been made to combat the salt issue in the lower Colorado, including the construction of a desalination plant at Yuma.{{cite web |url=http://www.usbr.gov/projects/Project.jsp?proj_Name=Colorado+River+Basin+Salinity+Control+Project |title=Colorado River Basin Salinity Control Project |publisher=U.S. Bureau of Reclamation |date=April 18, 2011 |access-date=February 21, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120228054940/http://www.usbr.gov/projects/Project.jsp?proj_Name=Colorado+River+Basin+Salinity+Control+Project |archive-date=February 28, 2012}} In 2011, the seven U.S. states agreed upon a "Plan of Implementation", which aims to reduce salinity by {{convert|644000|ST|t}} per year by 2030. In 2013, the Bureau of Reclamation estimated that around $32 million was spent each year to prevent around 1.2 million tons of salt from entering and damaging the Colorado River.{{Cite web |url=https://watershed.ucdavis.edu/education/classes/files/content/page/6%20Morford-Colorado_Basin_Salinity.pdf |title=Salinity in the Colorado River Basin |last=Morford |first=Scott |website=UC Davis Center for Watershed Sciences |access-date=February 23, 2017 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071325/https://watershed.ucdavis.edu/education/classes/files/content/page/6%20Morford-Colorado_Basin_Salinity.pdf |url-status=dead}}

Agricultural runoff containing pesticide residues has also been concentrated in the lower river in greater amounts. This has led to fish kills; six of these events were recorded between 1964 and 1968 alone.{{cite web |url=http://nepis.epa.gov/Exe/ZyNET.exe/91008B82.TXT?ZyActionD=ZyDocument&Client=EPA&Index=Prior+to+1976&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5Czyfiles%5CIndex%20Data%5C70thru75%5CTxt%5C00000008%5C91008B82.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=p%7Cf&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL |title=Technical Report: Pesticides in the Lower Colorado River |publisher=U.S. Environmental Protection Agency |work=Surveillance and Analysis Division, Report No. 002–73 |date=April 1973 |access-date=June 29, 2013 |archive-date=August 13, 2020 |archive-url=https://web.archive.org/web/20200813120226/https://nepis.epa.gov/Exe/ZyNET.exe/91008B82.TXT?ZyActionD=ZyDocument&Client=EPA&Index=Prior+to+1976&Docs=&Query=&Time=&EndTime=&SearchMethod=1&TocRestrict=n&Toc=&TocEntry=&QField=&QFieldYear=&QFieldMonth=&QFieldDay=&IntQFieldOp=0&ExtQFieldOp=0&XmlQuery=&File=D%3A%5Czyfiles%5CIndex%20Data%5C70thru75%5CTxt%5C00000008%5C91008B82.txt&User=ANONYMOUS&Password=anonymous&SortMethod=h%7C-&MaximumDocuments=1&FuzzyDegree=0&ImageQuality=r75g8/r75g8/x150y150g16/i425&Display=p%7Cf&DefSeekPage=x&SearchBack=ZyActionL&Back=ZyActionS&BackDesc=Results%20page&MaximumPages=1&ZyEntry=1&SeekPage=x&ZyPURL |url-status=live }} The pesticide issue is even greater in streams and water bodies near agricultural lands irrigated by the Imperial Irrigation District with Colorado River water. In the Imperial Valley, Colorado River water used for irrigation overflows into the New and Alamo rivers and into the Salton Sea. Both rivers and the sea are among the most polluted bodies of water in the United States, posing dangers not only to aquatic life but to contact by humans and migrating birds.{{cite web |url=http://www.waterboards.ca.gov/coloradoriver/about_us/about_region7.shtml |title=About Region 7 |publisher=California Environmental Protection Agency |work=Colorado River Basin Regional Water Quality Control Board |access-date=June 29, 2011 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071740/https://www.waterboards.ca.gov/coloradoriver/about_us/about_region7.html |url-status=live }}{{cite web |author=Polakovic, Gary |url=http://www.sci.sdsu.edu/salton/ASeaInTrouble.html |title=A Sea in Trouble |publisher=San Diego State University |work=The Press-Enterprise |access-date=June 29, 2011 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071730/http://www.sci.sdsu.edu/salton/ASeaInTrouble.html |url-status=live }} Pollution from agricultural runoff is not limited to the lower river; the issue is also significant in upstream reaches such as Colorado's Grand Valley, also a major center of irrigated agriculture.{{cite web |author1=Bauch, Nancy J. |author2=Spahr, Norman E. |url=http://pubs.usgs.gov/wri/wri00-4005/pdf/wrir00-4005.pdf |title=Pesticides in Surface Waters of the Upper Colorado River Basin, Colorado, 1996–98 |publisher=U.S. Geological Survey |work=Water-Resources Investigation Report 00-4005 |year=2000 |access-date=June 29, 2013 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819072006/https://pubs.usgs.gov/wri/wri00-4005/pdf/wrir00-4005.pdf |url-status=live }}

Large dams such as Hoover and Glen Canyon typically release water from lower levels of their reservoirs, resulting in stable and relatively cold year-round temperatures in long reaches of the river. The Colorado's average temperature once ranged from {{convert|85|F|C}} at the height of summer to near freezing in winter, but modern flows through the Grand Canyon, for example, rarely deviate significantly from {{convert|46|F|C}}.{{cite web |url=http://www.glencanyon.org/glen_canyon/grand-canyon |title=Grand Canyon |publisher=Glen Canyon Institute |access-date=February 21, 2012 |archive-url=https://web.archive.org/web/20120224091339/http://www.glencanyon.org/glen_canyon/grand-canyon |archive-date=February 24, 2012 |url-status=dead}} Changes in temperature regime have caused declines of native fish populations, and stable flows have enabled increased vegetation growth, obstructing riverside habitat.{{cite journal |title=Temperature Effects of Hypolimnial-Release Dams on Early Life Stages of Colorado River Basin Big-River Fishes |journal=Copeia |volume=2000 |date=May 8, 2000 |author1=Clarkson, Robert W. |author2=Childs, Michael R. |issue=2 |pages=402–412 |doi=10.1643/0045-8511(2000)000[0402:teohrd]2.0.co;2 |s2cid=85964507}} These flow patterns have also made the Colorado more dangerous to recreational boaters; people are more likely to die of hypothermia in the colder water, and the general lack of flooding allows rockslides to build up, making the river more difficult to navigate.{{sfn|Ghiglieri|Myers|2001|pp=194–214}}

In the 21st century, there has been renewed interest in restoring a limited water flow to the delta. In November 2012, the U.S. and Mexico reached an agreement, known as Minute 319, permitting Mexico storage of its water allotment in U.S. reservoirs during wet years, thus increasing the efficiency with which the water can be used. In addition to renovating irrigation canals in the Mexicali Valley to reduce leakage, this will make about {{convert|45000|acre.ft|m3}} per year available for release to the delta on average. The water will be used to provide both an annual base flow and a spring "pulse flow" to mimic the river's original snowmelt-driven regime.{{cite news |title=Colorado River Agreement to Help Restore Vanished Wetlands in Mexico |url=http://pri.org/stories/2012-11-26/colorado-river-agreement-help-restore-vanished-wetlands-mexico |first1=Jason |last1=Margolis |first2=Lisa |last2=Mullins |work=The World |publisher=Public Radio International |date=November 26, 2012 |access-date=November 2, 2013 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819072354/https://www.pri.org/stories/2012-11-26/colorado-river-agreement-help-restore-vanished-wetlands-mexico |url-status=live }}{{cite web |url=https://www.watereducation.org/userfiles/RiverReport_Winter13_web.pdf |title=Minute 319: Building on the Past to Provide for the Future |publisher=Water Education Foundation |work=River Report |date=2012–2013 |access-date=June 29, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20140501135538/https://www.watereducation.org/userfiles/RiverReport_Winter13_web.pdf |archive-date=May 1, 2014}} The first pulse flow, an eight-week release of {{convert|105000|acre.ft|m3}}, was initiated on March 21, 2014, with the aim of revitalising {{convert|2350|acre|hectare}} of wetland.{{cite magazine |title=Huge water pulse to bring Colorado river back from dead |url=https://www.newscientist.com/article/mg22129602.800-huge-water-pulse-to-bring-colorado-river-back-from-dead.html#.U3pqrijYJJQ |first=Hal |last=Hodson |magazine=New Scientist |date=March 12, 2014 |access-date=May 19, 2014 |archive-date=August 28, 2021 |archive-url=https://archive.today/20210828120731/https://www.newscientist.com/article/mg22129602-800-huge-water-pulse-to-bring-colorado-river-back-from-dead/?ignored=irrelevant%23.U3pqrijYJJQ |url-status=live }} This pulse reached the sea on May 16, 2014, marking the first time in 16 years that any water from the Colorado flowed into the ocean, and was hailed as "an experiment of historic political and ecological significance" and a landmark in U.S.–Mexican cooperation in conservation.{{cite magazine |title=Colorado river reaches sea for first time in decades |url=https://www.newscientist.com/article/dn25587#.U3pqsyjYJJQ |first=Hal |last=Hodson |magazine=New Scientist |date=May 19, 2014 |access-date=May 19, 2014 |archive-date=August 19, 2021 |archive-url=https://web.archive.org/web/20210819071944/https://www.newscientist.com/article/dn25587-colorado-river-reaches-sea-for-first-time-in-decades/#.U3pqsyjYJJQ |url-status=live }}{{cite news |last=Galvan |first=Astrid |date=May 19, 2014 |title=Colorado River Reaches Gulf |publisher=ABC and Associated Press |url=https://abcnews.go.com/US/wireStory/colorado-river-reaches-gulf-23753772 |url-status=dead |access-date=May 19, 2014 |archive-url=https://web.archive.org/web/20140519073738/https://abcnews.go.com/US/wireStory/colorado-river-reaches-gulf-23753772 |archive-date=May 19, 2014}} The pulse will be followed by the steady release of {{convert|52000|acre.ft|m3}} over the following three years, just a small fraction of its average flow before damming.{{needs update|date=February 2021}}

Image:Colorado River Runners.jpg

Famed for its dramatic rapids and canyons, the Colorado is one of the most well-known whitewater rivers in the United States, and its Grand Canyon section—run by more than 22,000 people annually{{cite news |author=Shorey, Ananda |agency=Associated Press |title=River May Be Getting More Rafters |newspaper=Deseret News |url=http://www.deseretnews.com/article/600099792/River-may-be-getting-more-rafters.html |location=Salt Lake City, Utah |date=December 23, 2004 |access-date=June 29, 2013 |archive-date=August 28, 2021 |archive-url=https://archive.today/20210828124928/https://www.deseret.com/2004/12/23/19868098/river-may-be-getting-more-rafters |url-status=dead }}—has been called the "granddaddy of rafting trips".{{sfn|Blond|Dunford|Schulte-Peevers|2008|p=87}} Grand Canyon trips typically begin at Lee's Ferry and take out at Diamond Creek or Lake Mead; they range from one to eighteen days for commercial trips and from two to twenty-five days for private trips.{{cite web |url=http://www.nps.gov/grca/planyourvisit/whitewater-rafting.htm |title=River Trips/Permits |publisher=U.S. National Park Service |work=Grand Canyon National Park |date=July 25, 2011 |access-date=February 23, 2012 |archive-date=June 29, 2012 |archive-url=https://web.archive.org/web/20120629125453/http://www.nps.gov/grca/planyourvisit/whitewater-rafting.htm |url-status=live }} Private (noncommercial) trips are extremely difficult to arrange because the National Park Service limits river traffic for environmental purposes; people who desire such a trip often have to wait more than 10 years for the opportunity.{{cite news |url=http://www.azcentral.com/travel/parks/grandcanyon/articles/2006/06/09/20060609story-rafting-CR.html |title=Rafting the Colorado River Through the Grand Canyon |work=azcentral.com |author=Jones, Melissa L. |date=June 9, 2006 |access-date=February 23, 2012 |archive-date=January 17, 2013 |archive-url=https://archive.today/20130117184920/http://www.azcentral.com/travel/parks/grandcanyon/articles/2006/06/09/20060609story-rafting-CR.html?nclick_check=1 |url-status=dead}}

Several other sections of the river and its tributaries are popular whitewater runs, and many of these are also served by commercial outfitters. The Colorado's Cataract Canyon and many reaches in the Colorado headwaters are even more heavily used than the Grand Canyon, and about 60,000 boaters run a single {{convert|4.5|mi|km|adj=on}} section above Radium, Colorado, each year. The upper Colorado also includes many of the river's most challenging rapids, including those in Gore Canyon, which is considered so dangerous that "boating is not recommended".{{cite web |url=http://www.blm.gov/co/st/en/fo/kfo/recreation_opportunities/rafting.html |title=Upper Colorado Special Recreation Management Area |publisher=U.S. Bureau of Land Management |date=September 29, 2011 |access-date=March 12, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20120306135619/http://www.blm.gov/co/st/en/fo/kfo/recreation_opportunities/rafting.html |archive-date=March 6, 2012}} Another section of the river above Moab, known as the Colorado "Daily" or "Fisher Towers Section", is the most visited whitewater run in Utah, with more than 77,000 visitors in 2011 alone.{{cite web |url=http://www.blm.gov/pgdata/etc/medialib/blm/ut/moab_fo/campgrounds/fee_accomplishment.Par.36998.File.dat/RecreationFeeAccountAccomplishmentsUT02Fy2011.pdf |title=Recreation Fee Account Accomplishments: Colorado River (Moab) |publisher=U.S. Bureau of Land Management |access-date=May 13, 2012 |url-status=dead |archive-url=https://web.archive.org/web/20121027155601/http://www.blm.gov/pgdata/etc/medialib/blm/ut/moab_fo/campgrounds/fee_accomplishment.Par.36998.File.dat/RecreationFeeAccountAccomplishmentsUT02Fy2011.pdf |archive-date=October 27, 2012}} The rapids of the Green River's Gray and Desolation Canyons{{sfn|Powell|p=345|2003}} and the less difficult "Goosenecks" section of the lower San Juan River are also frequently traversed by boaters.{{sfn|Armstead|p=143|1997}}

Eleven U.S. national parks—Arches, Black Canyon of the Gunnison, Bryce Canyon, Canyonlands, Capitol Reef, Grand Canyon, Mesa Verde, Petrified Forest, Rocky Mountain, Saguaro, and Zion—are in the watershed, in addition to many national forests, state parks, and recreation areas.{{cite web |url=http://www.npca.org/about-us/center-for-park-research/colorado_river_basin/Colorado-River-Report.pdf |title=National Parks of the Colorado River Basin: Water Resources, Threats and Economics |publisher=National Parks Conservation Association |date=April 2011 |access-date=June 24, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20120709093215/http://www.npca.org/about-us/center-for-park-research/colorado_river_basin/Colorado-River-Report.pdf |archive-date=July 9, 2012}} Hiking, backpacking, camping, skiing, and fishing are among the multiple recreation opportunities offered by these areas. Fisheries have declined in many streams in the watershed, especially in the Rocky Mountains, because of polluted runoff from mining and agricultural activities.{{cite web |author=Driver, Nancy E. |url=http://co.water.usgs.gov/nawqa/ucol/html/ofr_94_397_p16.html |title=Effects of Mining on Water Quality in Upper Colorado River Basin |publisher=U.S. Geological Survey |work=National Water-Quality Assessment Program – Upper Colorado River Basin Study Unit |date=April 15, 2013 |access-date=June 24, 2013 |url-status=dead |archive-url=https://web.archive.org/web/20150919054025/http://co.water.usgs.gov/nawqa/ucol/html/ofr_94_397_p16.html |archive-date=September 19, 2015}} The Colorado's major reservoirs are also heavily traveled summer destinations. Houseboating and water-skiing are popular activities on Lakes Mead, Powell, Havasu, and Mojave, as well as Flaming Gorge Reservoir in Utah and Wyoming, and Navajo Reservoir in New Mexico and Colorado. Lake Powell and surrounding Glen Canyon National Recreation Area received more than two million visitors per year in 2007,{{cite web |url=http://www.nps.gov/glca/parknews/upload/Expanded%20Action%20Plan.pdf |title=Glen Canyon National Recreation Area Expanded Zebra Mussel Action Plan |publisher=U.S. National Park Service |work=Glen Canyon National Recreation Area |access-date=February 23, 2012 |archive-date=September 16, 2012 |archive-url=https://web.archive.org/web/20120916200101/http://www.nps.gov/glca/parknews/upload/Expanded%20Action%20Plan.pdf |url-status=live }} while nearly 7.9 million people visited Lake Mead and the Lake Mead National Recreation Area in 2008.{{cite web |url=http://www.nps.gov/lake/parknews/lake-mead-proves-popular-during-economic-downturn.htm |title=Lake Mead Proves Popular During Economic Downturn |publisher=U.S. National Park Service |work=Lake Mead National Recreation Area |date=February 5, 2009 |access-date=February 23, 2012 |archive-date=September 12, 2010 |archive-url=https://web.archive.org/web/20100912062847/http://www.nps.gov/lake/parknews/lake-mead-proves-popular-during-economic-downturn.htm |url-status=live }} Colorado River recreation employs some 250,000 people and contributes $26 billion each year to the Southwest economy.{{cite news |url=http://www.deseretnews.com/article/865584776/Worst-Colorado-River-drought-in-century-prompts-feds-to-cut-releases-from-Lake-Powell.html?pg=all |author=O'Donoghue, Amy Joi |title=Worst Colorado River drought in century prompts feds to cut releases from Lake Powell |work=Deseret News |date=August 16, 2013 |access-date=August 22, 2013 |archive-date=November 12, 2020 |archive-url=https://web.archive.org/web/20201112020213/https://www.deseret.com/2013/8/16/20524101/worst-colorado-river-drought-in-century-prompts-feds-to-cut-releases-from-lake-powell |url-status=dead}}

{{Portal|Arizona|California|Colorado|Mexico|United States|Utah}}

• Colorado River Delta
• Colorado Desert
• List of Colorado River rapids and features
• List of dams in the Colorado River system
• List of largest reservoirs in the United States
• List of longest rivers of Mexico
• List of longest rivers of the United States (by main stem)
• London Bridge (Lake Havasu City)
• Moab uranium mill tailings pile
• Upper Colorado River Endangered Fish Recovery Program

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• {{cite book |last=Leuchtenburg |first=William Edward |title=American Places: Encounters with History |url=https://archive.org/details/americanplaces00will |url-access=registration |publisher=Oxford University Press |year=2000 |isbn=978-0-19-513026-3}}
• {{cite book |last=Leopold |first=Luna Bergere |title=A View of the River |publisher=Harvard University Press |year=1994 |isbn=978-0-674-93732-1}}
• {{cite book |last=Lindberg |first=Eric |title=Colorado Off the Beaten Path: A Guide to Unique Places |publisher=Globe Pequot |year=2009 |isbn=978-0-7627-5024-5 |url=https://archive.org/details/coloradooffbeate0000lind}}
• {{cite book |last=Lingenfelter |first=Richard E. |title=Steamboats on the Colorado River, 1852–1916 |publisher=University of Arizona Press |year=1978 |isbn=978-0-8165-0650-7}}
• {{cite book |last=Logan |first=Michael F. |title=Desert Cities: The Environmental History of Phoenix and Tucson |publisher=University of Pittsburgh Press |year=2006 |isbn=978-0-8229-4294-8}}
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• {{cite book |last=Prisciantelli |first=Tom |title=Spirit of the American Southwest: Geology, Ancient Eras and Prehistoric People, Hiking Through Time |publisher=Sunstone Press |year=2002 |isbn=978-0-86534-354-2}}
• {{cite book |last=Pritzker |first=Barry |title=Native Americans: An Encyclopedia of History, Culture, and Peoples |publisher=ABC-CLIO |volume=1 |year=1998 |isbn=978-0-87436-836-9 |url=https://archive.org/details/nativeamericanse00barr}}
• {{cite book |last=Reisner |first=Marc |title=Cadillac Desert |publisher=Penguin |year=1993 |isbn=978-0-14-017824-1 |url=https://archive.org/details/cadillacdesertam00reis}}
• {{cite book |last=Rolle |first=Andrew |title=John Charles Fremont: Character As Destiny |publisher=University of Oklahoma Press |year=1999 |isbn=978-0-8061-3135-1}}
• {{cite book |last=Schmidt |first=Jeremy |title=Grand Canyon National Park: A Natural History Guide |publisher=Houghton Mifflin Harcourt |year=1993 |isbn=978-0-395-59932-7 |url=https://archive.org/details/grandcanyonnatio00schm}}
• {{cite book |last=Van Cott |first=John W. |title=Utah Place Names: A Comprehensive Guide to the Origins of Geographic Names |publisher=University of Utah Press |year=1990 |isbn=978-0-87480-345-7}}
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• {{cite book |title=The Colorado River: Origin and Evolution |publisher=Grand Canyon Association |year=2001 |isbn=978-0-938216-79-7 |last1=Young |first1=Richard A. |last2=Spamer |first2=Earle E.}}

{{refend}}

{{div col}}

• Darrah, William Culp, Ralph V. Chamberlin, and Charles Kelly, editors. (2009). The Exploration of the Colorado River in 1869 and 1871–1872: Biographical Sketches and Original Documents of the First Powell Expedition of 1869 and the Second Powell Expedition of 1871–1872. {{ISBN|978-0-87480-963-3}}.
• {{cite book |author=DeBuys, William |title=A Great Aridness: Climate Change and the Future of the American Southwest |publisher=Oxford University Press |year=2011 |isbn=978-0-19-977892-8}}
• Fleck, Richard F., editor. (2000). A Colorado River Reader. {{ISBN|978-0-87480-647-2}}.
• Fowler, Don D., editor. (2012). Cleaving an Unknown World: The Powell Expeditions and the Scientific Exploration of the Colorado Plateau. {{ISBN|978-1-60781-146-6}}.
• Gregory, Herbert E., William Culp Darrah, and Charles Kelly, editors. (2009). The Exploration of the Colorado River and the High Plateaus of Utah by the Second Powell Expedition of 1871–1872. {{ISBN|978-0-87480-964-0}}.
• {{cite book |author=Martin, Russell |title=A Story That Stands Like A Dam: Glen Canyon and the Struggle for the Soul of the West |publisher=Henry Holt and Company |edition=1 |isbn=978-0-8050-0822-7 |year=1990 |url=https://archive.org/details/storythatstandsl00russ}}
• {{Citation | last = McPherson | first = Robert | title = Utah History Encyclopedia | publisher = University of Utah Press | year = 1994 | chapter = Colorado River | chapter-url = https://www.uen.org/utah_history_encyclopedia/c/COLORADO_RIVER.shtml | url = https://www.uen.org/utah_history_encyclopedia/ | archive-url = https://web.archive.org/web/20231208224550/https://www.uen.org/utah_history_encyclopedia/c/COLORADO_RIVER.shtml | archive-date = December 8, 2023 | isbn =9780874804256 | access-date = April 12, 2024}}
• Schmidt, John C., Lindsey Bruckerhoff, Homa Salehabadi, and Jian Wang. The Colorado River. In Avijit Gupta, ed., Large Rivers: Geomorphology and Management. 2nd Ed. Hoboken, N.J.: Wiley-Blackwell, pp. 253–319. ISBN 9781119412601
• {{cite book |author=Summitt, April R. |title=Contested Waters: An Environmental History of the Colorado River |publisher=University Press of Colorado |location=Boulder |isbn=978-1-60732-201-6 |year=2013}}

{{div col end}}

{{sister project links|auto=yes}}

• [https://web.archive.org/web/20131104165706/http://www.crwua.org/colorado-river/uses/agriculture Agriculture in the Colorado River Basin]: Colorado River Water Users Association
• [http://awesome.good.is.s3.amazonaws.com/transparency/web/1009/colorado-river-2/flat.html Colorado River Water Allocations by State]: GOOD Infographics
• [https://web.archive.org/web/20120219044038/http://www.usbr.gov/uc/feature/drought.html Drought in the Upper Colorado River Basin]: U.S. Bureau of Reclamation
• [https://web.archive.org/web/20130806050034/http://www.pacinst.org/reports/co_river_ag_2013/map/ Irrigation Water Withdrawals in the Colorado River Basin]: Pacific Institute
• [https://web.archive.org/web/20120203155107/http://livingrivers.org/ Living Rivers: Colorado Riverkeeper]
• [http://water-data.com/ Water Level Data for Major Colorado River Reservoirs]: water-data.com
• [https://www.nytimes.com/2012/02/15/opinion/where-the-colorado-river-runs-dry.html?_r=2&ref=opinion Where the Colorado Runs Dry]: The New York Times
• 1854 report from the U.S. Army Corps of Topographical Engineers: [https://www.biodiversitylibrary.org/item/124223#page/9/mode/1up Report of an expedition down the Zuni and Colorado Rivers]
• [https://www.propublica.org/series/killing-the-colorado Killing the Colorado] – ProPublica
• Lawrence Pratt Collection Concerning Arizona v. California and the Colorado River. Yale Collection of Western Americana, Beinecke Rare Book and Manuscript Library.

{{Colorado River system}}

{{Grand Canyon|state=autocollapse}}

{{United States topics}}

{{authority control}}

Category:Border rivers

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