Lake Atna

In 1898, Frank Charles Schrader undertook a study of the Copper River Basin for the United States Geological Survey (USGS). Based on sedimentary evidence, he concluded that there was a possibility of a large body of standing water being responsible for the deposits and that this could have been an arm of the sea.{{cite report |author=Schrader, F. C. |year=1900 |title=Twentieth Annual report of the United States Geological Survey, 1898–1899 |url=https://pubs.usgs.gov/ar/20-7/report.pdf |publisher=Government Printing House |series=Part VII – Explorations in Alaska in 1898 |section=A Reconnaissance of a Part of Prince William Sound and the Copper River District, Alaska, in 1898 |page=402 |access-date=19 January 2017 |archive-date=31 January 2017 |archive-url=https://web.archive.org/web/20170131200143/https://pubs.usgs.gov/ar/20-7/report.pdf |url-status=live }} Alongside A. C. Spencer in 1901, he concluded – contrary to his earlier hypothesis – that these deposits occurred only in limited areas. This conclusion was supported by Walter Curran Mendenhall in 1905, who had studied the Pleistocene deposits in the central region of the basin.{{harvnb|Ferrians|1989|p=85}}

In 1954, Fred Howard Moffit noted that topographic conditions were favorable for the possibility of a large lake, but that specific evidence was lacking at that time.{{cite journal |last1=Moffit |first1=Fred Howard |title=Geology of the Eastern Part of the Alaska Range and Adjacent Area |journal=Geological Survey Bulletin |year=1954 |volume=989-D |page=159 |url=http://pubs.dggsalaskagov.us/webpubs/usgs/b/text/b0989d.pdf |access-date=19 January 2017 |archive-date=31 January 2017 |archive-url=https://web.archive.org/web/20170131123226/http://pubs.dggsalaskagov.us/webpubs/usgs/b/text/b0989d.pdf |url-status=live }} By 1957, geologists Oscar J. Ferrians and H.R. Schmoll concluded the basin had been resident to a large proglacial lake during the Wisconsin glaciation.{{cite journal |last1=Ferrian |first1=Oscar J. Jr. |title=Extensive proglacial lake of Wisconsin Age in the Copper River Basin |journal=Geological Society of America Bulletin |year=1957 |volume=68 |issue=12, part 2 |page=1726}} The lake was named Lake Atna by geologist D. J. Nichols of the USGS in 1965.

File:LakeAtnaDeposits.jpg lacustrine, alluvial, and glacial deposits that fill the Copper River Basin. Most of the section seen in this view consists of finely laminated to indistinctly bedded sand, silt, and clay, with or without coarser material, that was deposited in glacial Lake Atna.]]

During the Miocene Epoch, the region saw significant tectonic uplift through which the ancestral Copper River maintained its course. Sometime before 40 ka, the basin was free of water and ice, and covered in spruce forest.{{harvnb|Bennett|2002|p=2240}} During the Wisconsin glaciation, a multitude of glaciers extended into and blocked drainage exits from the Copper River Basin. The lake formed at least as early as 58 ka,{{harvnb|Ferrians|1989|p=87}} during the Pleistocene period as a result of glacial damming along the middle and lower valleys of the river, impounding the lake in the Copper River Basin.{{harvnb|Winkler|2000|p=121}}

The highest surface level the lake achieved was {{convert|975|m|abbr=on}}, based on sedimentation in the northwestern area of the basin.{{harvnb|Wiedmer|Montgomery|Gillespie|Greenberg|2010|p=414}} Multiple strand lines exist in the basin indicating sustained lake levels at a maximum of {{convert|914|m|abbr=on}} above modern sea level and other lower levels including {{convert|750|m|abbr=on}}, {{convert|700|m|abbr=on}}, {{convert|580|m|abbr=on}}, {{convert|550|m|abbr=on}} and {{convert|490|m|abbr=on}} above modern sea level.{{harvnb|Winkler|2000|p=81}} At its greatest extent, the lake surface area was approximately half the size of modern-day Lake Ontario, and possibly much larger.

During its early formation, the lake likely had no permanent outlet. The damming glaciers that created the lake became large enough that the lake was endorheic, though it may have periodically topped its glacial dams. The final draining and disappearance of the lake likely happened no later than approximately 9.4 ka through the Copper River Valley. Isostatic rebound effects of departing ice sheets and the draining of Lake Atna from the basin remain uncertain, as detailed study of the shorelines in the basin has not been conducted.

In 2006, John Jangala, an archaeologist with the Bureau of Land Management indicated that Lake Atna may have been a remnant of an earlier lake called Lake Susitna.{{cite web |last1=Ned |first1=Rozell |title=Searching for signs of Lake Atna |url=http://www.sitnews.us/0206news/020906/020906_ak_science.html |website=SitNews |access-date=19 January 2017 |archive-date=23 July 2012 |archive-url=https://web.archive.org/web/20120723213258/http://www.sitnews.us/0206news/020906/020906_ak_science.html |url-status=live }} This was contradicted by Michael Wiedmer in 2011 who stated that no evidence exists for an independent Lake Susitna.{{cite journal |last1=Wiedmer |first1=Michael |last2=Montgomery |first2=David R. |last3=Gillespie |first3=Alan R. |last4=Greenberg |first4=Harvey |title=Reply to the comments of Reger, Lowell, and Evenson on "Late Quaternary megafloods from Glacial Lake Atna, Southcentral Alaska, U.S.A." |journal=Quaternary Research |year=2011 |volume=75 |issue=1 |page=303 |doi=10.1016/j.yqres.2010.09.003 |bibcode=2011QuRes..75..303W |s2cid=128892602 |url=http://ac.els-cdn.com/S0033589410001134/1-s2.0-S0033589410001134-main.pdf?_tid=3492ce12-d905-11e6-8f06-00000aacb362&acdnat=1484252886_0f51baccecc0400723e441e3a25fb653 |access-date=19 January 2017}} The detailed history of the lake and its extent over time is a highly complex problem, as further investigation is required.{{harvnb|Smith|2019}}

File:LakeAtnaRelatedGeography.jpg

In 2005, Michael Wiedmer, then a biologist at the Alaska Department of Fish and Game, was shown the carcass of a pygmy whitefish which had been collected from Lake George. This lake sits near the face of Knik Glacier, some distance from the Copper River Basin area. Pygmy whitefish are known to exist in remnant lakes of Lake Atna.{{cite web |title=Pygmy Whitefish |url=https://www.adfg.alaska.gov/static/species/speciesinfo/_aknhp/Pygmy_Whitefish.pdf |website=Alaska Department of Fish and Game |publisher=State of Alaska |access-date=19 January 2017 |archive-date=3 February 2017 |archive-url=https://web.archive.org/web/20170203091335/http://www.adfg.alaska.gov/static/species/speciesinfo/_aknhp/Pygmy_Whitefish.pdf |url-status=live }} This offered a clue as to a water connection that may have existed at some time in the past between Lake Atna and Lake George. Wiedmer also observed the presence of large, symmetrical hills in the Matanuska Valley, possibly of fluvial origin. This led Wiedmer to investigate the possibility of a megaflood down the valley from ancient Lake Atna.{{cite web |last1=Rozell |first1=Ned |title=Spillways of an ancient Alaska lake |url=http://www.gi.alaska.edu/alaska-science-forum/spillways-ancient-alaska-lake |website=University of Alaska Fairbanks – Geophysical Institute |publisher=University of Alaska Fairbanks |access-date=19 January 2017 |archive-date=31 January 2017 |archive-url=https://web.archive.org/web/20170131200623/http://www.gi.alaska.edu/alaska-science-forum/spillways-ancient-alaska-lake |url-status=live }}{{cite news |last1=Stricherz |first1=Vince |title=Research shows part of Alaska inundated by ancient megafloods |url=http://www.washington.edu/news/2010/04/29/research-shows-part-of-alaska-inundated-by-ancient-megafloods-2/ |access-date=19 January 2017 |work=UW Today |publisher=University of Washington |date=29 April 2010 |archive-date=1 February 2017 |archive-url=https://web.archive.org/web/20170201234439/http://www.washington.edu/news/2010/04/29/research-shows-part-of-alaska-inundated-by-ancient-megafloods-2/ |url-status=live }}

Wiedmer's resulting research was published in a 2010 paper titled "Late Quaternary megafloods from Glacial Lake Atna, Southcentral Alaska, U.S.A.", with co-authors from the Quaternary Research Center at the University of Washington.{{harvnb|Wiedmer|Montgomery|Gillespie|Greenberg|2010}} Wiedmer's research suggested the possibility of Lake Atna being a serial generator of megafloods from glacial dam failures. One such possible flood originated from a catastrophic failure of a glacial dam {{convert|61|m|abbr=on}} in height at Tahneta Pass, located at the eastern end of Matanuska Valley. Wiedmer's maximum estimates of this flood place its size at a 2.0–3.3 × 10⁶{{Spaces}}m³s⁻¹ (2.0–3.3 million cubic meters per second) discharge rate. The velocity estimates of this flood range from a minimum of {{convert|13|m/s|mph|abbr=on}} to {{convert|57|m/s|mph|abbr=on}}.{{harvnb|Wiedmer|Montgomery|Gillespie|Greenberg|2010|p=421}} The flood released a maximum of {{convert|1400|km3|abbr=on}} of water into the Wasilla region over the span of a week. The intensity of the release was such that it created dunes {{convert|34|m|abbr=on}} in height, with crests of the dunes {{convert|0.8|km|abbr=on}} and more apart. The dunes still exist today.

Other glacial dam outbursts may have occurred down the Susitna River, through Mentasta Pass into the Tok River,{{cite conference |url=https://gsa.confex.com/gsa/2010AM/finalprogram/abstract_177919.htm |title=Model for Late Wisconsinan Massive Outburst Flooding, Tok River Valley, Northeastern Alaska Range |last1=Hubbard |first1=Trent D. |last2=Reger |first2=Richard D. |year=2010 |conference=The Geological Society of America (GSA) 2010 Denver Annual Meeting |conference-url=https://gsa.confex.com/gsa/2010AM/finalprogram/index.html |access-date=19 January 2017 |archive-date=31 January 2017 |archive-url=https://web.archive.org/web/20170131195734/https://gsa.confex.com/gsa/2010AM/finalprogram/abstract_177919.htm |url-status=live }}{{harvnb|Wiedmer|Montgomery|Gillespie|Greenberg|2010|p=423}} and down the Copper River itself. The Susitna River outburst may have been responsible for a flood three to four times more intense than the Tahneta Pass outburst, at a 7.0–11.3 × 10^6{{Spaces}}m³s⁻¹ discharge rate. This flood may have discharged nearly twice as much water as the Tahneta Pass outburst. The postulated glacial dam height for this flood was {{convert|346|m|abbr=on}}.

File:CopperRiverBasin.jpg towards Mt. Drum. Lake Atna covered this basin.]]

Diamicton, glacial, glaciolacustrine, and lacustrine origin deposits exist in many areas of the Copper River Basin.{{harvnb|Bennett|2002|p=2237}} All major rivers present today occupy narrow valleys that have cut up to {{convert|137|m|abbr=on}} into the bottom of the basin. Lacustrine sediments of the lake are visible in many river bluffs throughout the basin. Several extant lakes in the basin are remnants of the lake, including Tazlina Lake, Klutina Lake, and Tonsina Lake. Much of the unconsolidated fill of the basin is in permafrost, down to a maximum depth of {{convert|75|m|abbr=on}} in the central basin area. The postulated Matanuska Valley flood produced by Lake Atna 17,000 years ago may have been responsible for the deposition of a thin stratum of clay and silt grains in the Anchorage area. The collapse of this stratum caused catastrophic landslides in the Anchorage area during the 1964 Alaska earthquake contributing to the collapse of structures built above it.{{harvnb|Wiedmer|Montgomery|Gillespie|Greenberg|2010|p=417}}{{cite news |last1=Stricherz |first1=Vince |title=Research shows part of Alaska inundated by ancient megafloods |url=http://www.washington.edu/news/2010/04/29/research-shows-part-of-alaska-inundated-by-ancient-megafloods-2/ |access-date=19 January 2017 |work=UW Today |publisher=University of Washington |date=29 April 2010}}

• List of prehistoric lakes

{{reflist}}

{{refbegin}}

• {{Citation |last1=Wiedmer |first1=Michael |last2=Montgomery |first2=David R. |last3=Gillespie |first3=Alan R. |last4=Greenberg |first4=Harvey |publication-date=2010 |title=Late Quaternary megafloods from Glacial Lake Atna, Southcentral Alaska, U.S.A. |journal=Quaternary Research |year=2010 |volume=73 |issue=3 |publisher=Elsevier Inc. |pages=413–424 |doi=10.1016/j.yqres.2010.02.005 |bibcode=2010QuRes..73..413W |s2cid=129855432 |url=http://gis.ess.washington.edu/grg/publications/pdfs/Wiedmer_etal_2010.pdf |access-date=19 January 2017 |archive-date=15 December 2017 |archive-url=https://web.archive.org/web/20171215160220/http://gis.ess.washington.edu/grg/publications/pdfs/Wiedmer_etal_2010.pdf |url-status=live }}
• {{Citation |last=Ferrians |first=Oscar J. Jr. |publication-date=1989 |title=Glacial Lake Atna, Copper River Basin, Alaskan |journal=U.S. Geological Survey Circular |volume=1026 |publisher=U.S. Geological Survey |pages=85–88 |url=https://pubs.usgs.gov/circ/1989/1026/report.pdf |access-date=19 January 2017 |archive-date=31 January 2017 |archive-url=https://web.archive.org/web/20170131195922/https://pubs.usgs.gov/circ/1989/1026/report.pdf |url-status=live }}
• {{Citation |last=Bennett |first=Matthew R. |publication-date=2002 |title=Facies architecture within a regional glaciolacustrine basin: Copper River, Alaska |journal=Quaternary Science Reviews |year=2002 |volume=21 |issue=20–22 |publisher=Elsevier Inc. |pages=2237–2279 |doi=10.1016/S0277-3791(02)00027-6 |bibcode=2002QSRv...21.2237B |url=http://ac.els-cdn.com/S0277379102000276/1-s2.0-S0277379102000276-main.pdf?_tid=57a16cfa-d848-11e6-a6ef-00000aab0f26&acdnat=1484171770_982f32ca98e603a04e0233a9317a3877 |access-date=19 January 2017 |format=PDF }}
• {{Citation |last=Winkler |first=Gary R. |publication-date=2000 |title=A Geologic Guide to Wrangell–Saint EliasNational Park and Preserve, Alaska |publisher=U.S. Department of the Interior, U.S. Geological Survey |url=https://www.nps.gov/wrst/learn/nature/upload/A-Geologic-Guide-color-version-for-web.pdf |access-date=19 January 2017 |archive-date=20 February 2017 |archive-url=https://web.archive.org/web/20170220223838/https://www.nps.gov/wrst/learn/nature/upload/A-Geologic-Guide-color-version-for-web.pdf |url-status=live }}
• {{Citation |last=Smith |first=Gerad M. |publication-date=2019 |title=Geoarchaeology of Glacial Lakes Susitna and Atna |publisher=Alaska Journal of Anthropology |url=https://www.researchgate.net/publication/336721960 |access-date=2 November 2019 |format=PDF |archive-date=1 July 2021 |archive-url=https://web.archive.org/web/20210701075339/https://www.researchgate.net/publication/336721960_Geoarchaeology_of_Glacial_Lakes_Susitna_and_Atna |url-status=live }}

{{refend}}

{{Pleistocene Lakes and Seas}}

{{authority control}}

{{DEFAULTSORT:Atna, Lake}}

Category:Former lakes of the United States

Category:Geology of Alaska

Category:Glacial lakes of the United States

Category:Lakes of Alaska

Category:Megafloods

Category:Natural history of Alaska

Category:Pleistocene United States

Category:Proglacial lakes