Hydrologic unit system (United States)

{{short description|Watershed tracking system}}

{{Use mdy dates|date=April 2024}} {{Use American English|date=April 2024}}

Image:HUC subregions.pngs updated boundaries to include the ocean as well as the portions of the basins that cross international borders]]

For the use of hydrologists, ecologists, and water-resource managers in the study of surface water flows in the United States, the United States Geological Survey created a hierarchical system of hydrologic units.

File:Infographic explaining the hierarchy of the United States hydrologic unit system.png

Originally a four-tier system divided into regions, sub-regions, accounting units, and cataloging units, each unit was assigned a unique Hydrologic Unit Code (HUC). As first implemented the system had 21 regions, 221 subregions, 378 accounting units, and 2,264 cataloging units.{{cite journal|last=Seaber, Paul R., F. Paul Kapanos, and George L. Knapp|title=Hydrologic Unit Maps|journal=United States Geological Survey Water-supply Papers|year=1987|number=2294|pages=i-iii, 1–63}}{{cite web|title=Hydrologic Unit Maps - What are Hydrologic Units?|url=http://water.usgs.gov/GIS/huc.html|publisher=USGS|access-date=2010-10-27|archive-date=2023-01-15|archive-url=https://web.archive.org/web/20230115224722/https://water.usgs.gov/GIS/huc.html|url-status=live}} {{PD-notice}} Over time the system was changed and expanded.{{cite web |title= Overview and History of Hydrologic Units and the Watershed Boundary Dataset (WBD) |date= 11 January 2022 |publisher= Natural Resources Conservation Service |url= http://www.ncgc.nrcs.usda.gov/products/datasets/watershed/history.html |access-date= 18 March 2023 |archive-date= 24 July 2011 |archive-url= https://web.archive.org/web/20110724091345/http://www.ncgc.nrcs.usda.gov/products/datasets/watershed/history.html |url-status= live }} {{PD-notice}} As of 2010 there are six levels in the hierarchy, represented by hydrologic unit codes from 2 to 12 digits long, called regions, subregions, basins, subbasins, watersheds, and subwatersheds. The table below describes the system's hydrologic unit levels and their characteristics, along with example names and codes.{{cite web |title= Watershed Boundary Dataset (WBD) Facts |date= 11 January 2022 |publisher= Natural Resources Conservation Service |url= http://www.ncgc.nrcs.usda.gov/products/datasets/watershed/facts.html |access-date= 18 March 2023 |archive-date= 24 July 2011 |archive-url= https://web.archive.org/web/20110724085742/http://www.ncgc.nrcs.usda.gov/products/datasets/watershed/facts.html |url-status= live }} {{PD-notice}}

class="wikitable" border="1" style="margin: 1em auto 1em auto;"
valign="top" ! scope="col" | Name ! scope="col" | Level ! scope="col" | Digits ! scope="col" | Average size (square miles) ! scope="col" | Number of HUs (approximate) ! scope="col" | Example name ! scope="col" | Example code (HUC)
valign="top" ! scope="row" | Region | align="center" | 1	align="center" | 2	align="right" | 177,560	align="right" | 21	align="center" | Pacific Northwest	17
valign="top" ! scope="row" | Subregion | align="center" | 2	align="center" | 4	align="right" | 16,800	align="right" | 222	align="center" | Lower Snake	1706
valign="top" ! scope="row" | Basin | align="center" | 3	align="center" | 6	align="right" | 10,596	align="right" | 370	align="center" | Lower Snake	170601
valign="top" ! scope="row" | Subbasin | align="center" | 4	align="center" | 8	align="right" | 700	align="right" | 2,200	align="center" | Imnaha River	17060102
valign="top" ! scope="row" | Watershed | align="center" | 5	align="center" | 10	align="right" | 227 (40,000–250,000 acres)	align="right" | 22,000	align="center" | Upper Imnaha River	1706010201
valign="top" ! scope="row" | Subwatershed | align="center" | 6	align="center" | 12	align="right" | 40 (10,000–40,000 acres)	align="right" | 160,000	align="center" | North Fork Imnaha River	170601020101

The original delineation of units, down to subbasins (cataloging units), was done using 1:250,000 scale maps and data. The newer delineation work on watersheds and subwatersheds was done using 1:24,000 scale maps and data. As a result, the subbasin boundaries were changed and adjusted in order to conform to the higher resolution watersheds within them. Changes to subbasin boundaries resulted in changes in area sizes. Therefore, older data using "cataloging units" may differ from newer, higher resolution data using "subbasins".{{cite web|url=https://pubs.usgs.gov/tm/11/a3/pdf/tm11-a3.pdf|title=Federal guidelines, requirements, and procedures for the national Watershed Boundary Dataset: U.S. Geological Survey Techniques and Methods 11–A3|year=2009|publisher=Natural Resources Conservation Service and United States Geological Survey|access-date=4 November 2010|archive-date=14 September 2015|archive-url=https://web.archive.org/web/20150914202437/http://pubs.usgs.gov/tm/11/a3/pdf/tm11-a3.pdf|url-status=live}}

Image:Huc region-edit.png, 02 Mid Atlantic, 03 South Atlantic Gulf, 04 Great Lakes, 05 Ohio, 06 Tennessee, 07 Upper Mississippi, 08 Lower Mississippi, 09 Souris-Red-Rainy, 10 Missouri, 11 Arkansas-White-Red, 12 Texas-Gulf, 13 Rio Grande, 14 Upper Colorado, 15 Lower Colorado, 16 Great Basin, 17 Pacific Northwest, 18 California, 19 Alaska, 20 Hawaii, 21 Caribbean.]]

The regions (1st level hydrologic units) are geographic areas that contain either the drainage area of a major river, such as the Missouri region, or the combined drainage areas of a series of rivers, such as the Texas–Gulf region. Each subregion includes the area drained by a river system, a reach of a river and its tributaries in that reach, a closed basin or basins, or a group of streams forming a coastal drainage area.{{cite web |title= Hydrologic Unit Maps, U.S. Geological Survey Water-Supply Paper 2294 |last1= Seaber |first1= Paul R. |last2= Kapinos |first2= F. Paul |last3= Knapp |first3= George L. |publisher= United States Geological Survey |url= http://pubs.usgs.gov/wsp/wsp2294/pdf/wsp_2294.pdf |access-date= 3 November 2010 |archive-date= 29 June 2011 |archive-url= https://web.archive.org/web/20110629125633/http://pubs.usgs.gov/wsp/wsp2294/pdf/wsp_2294.pdf |url-status= live }} Regions receive a two-digit code. The following levels are designated by the addition of another two digits.{{cite web|url=https://www.nrcs.usda.gov/programs/rwa/Watershed_HU_HUC_WatershedApproach_defined_6-18-07.pdf |title=Watersheds, Hydrologic Units, Hydrologic Unit Codes, Watershed Approach, and Rapid Watershed Assessments |publisher=USDA |access-date=2010-10-27 |url-status=dead|archive-url=https://web.archive.org/web/20101012233329/http://www.nrcs.usda.gov/programs/rwa/Watershed_HU_HUC_WatershedApproach_defined_6-18-07.pdf |archive-date=2010-10-12 }}

The hierarchy was designed and the units subdivided so that almost all the subbasins (formerly called cataloging units) are larger than {{convert|700|sqmi|km2}}. Larger closed basins were subdivided until their subunits were less than 700 square miles. The 10-digit watersheds were delineated to be between 40,000 and 250,000 acres in size, and the 12-digit subwatersheds between 10,000 and 40,000 acres. In addition to the hydrologic unit codes, each hydrologic unit was assigned a name corresponding to the unit's principal hydrologic feature or to a cultural or political feature within the unit. The boundaries of the hydrologic units usually correspond to drainage basins with some exceptions; for example, subregion 1711, called "Puget Sound", includes all U.S. drainage into not only Puget Sound but also the Strait of Georgia, Strait of Juan de Fuca, and the Fraser River.{{cite web |url= http://wa.water.usgs.gov/data/wuse/huc.names.txt |title= List Hydrologic Unit Codes (HUCs) - USGS Washington |publisher= USGS |access-date= 19 July 2011 |archive-date= 7 June 2014 |archive-url= https://web.archive.org/web/20140607005745/http://wa.water.usgs.gov/data/wuse/huc.names.txt |url-status= live }} Also, region and subregion boundaries end at the U.S. international boundary.

The various subdivisions of this system are not necessarily synonymous with watersheds for a number of reasons. As one analysis put it: "The hydrologic unit framework is in fact composed mostly of watersheds and pieces of watersheds. The pieces include units that drain to segments of streams, remnant areas, noncontributing areas, and coastal or frontal units that can include multiple watersheds draining to an ocean or large lake. Hence, half or more of the hydrologic units are not watersheds as the name of the framework Watershed Boundary Dataset (WBD) implies. Nonetheless, hydrologic units and watersheds are commonly treated as synonymous, and this misapplication and misunderstanding can have some serious scientific and management consequences."{{Cite journal |last1=Omernik |first1=James M. |last2=Griffith |first2=Glenn E. |last3=Hughes |first3=Robert M. |last4=Glover |first4=James B. |last5=Weber |first5=Marc H. |date=September 2017 |title=How Misapplication of the Hydrologic Unit Framework Diminishes the Meaning of Watersheds |journal=Environmental Management |language=en |volume=60 |issue=1 |pages=1–11 |doi=10.1007/s00267-017-0854-z |issn=0364-152X |pmc=6145848 |pmid=28378091}}

Aquifers of the United States are organized by national principal aquifer codes and names assigned by the National Water Information System (NWIS). Aquifers are identified by a geohydrologic unit code (a three-digit number related to the age of the formation) followed by a 4 or 5 character abbreviation for the geologic unit or aquifer name.{{Cite web |title=National Aquifer Code Reference List |url=https://www.usgs.gov/mission-areas/water-resources/science/national-aquifer-code-reference-list#overview |access-date=2023-03-19 |website=U.S. Geological Survey |date= March 8, 2021 |archive-date=2023-03-19 |archive-url=https://web.archive.org/web/20230319204944/https://www.usgs.gov/mission-areas/water-resources/science/national-aquifer-code-reference-list#overview |url-status=live }}

{{gallery | title = | mode = packed

|File:HUC 4 digit codes national map for demonstration instruction 01.gif |Water resource subregions (4-digit codes)

|File:HUC 6 digit codes national map for demonstration instruction 03.gif |Water resource basins (6-digit codes)

|File:HUC 8 digit codes national map for demonstration instruction 04.gif |Water resource subbasins (8-digit codes)

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