Hydrologic Unit Codes (HUCs) Explained

Hydrologic Unit Codes (HUCs)

Watersheds are delineated by USGS using a nationwide system based on surface hydrologic features. This system divides the country into 21 regions (2-digit), 222 subregions (4-digit), 352 accounting units (6-digit), and 2,262 cataloguing units (8-digit). A hierarchical hydrologic unit code (HUC) consisting of 2 digits for each level in the hydrologic unit system is used to identify any hydrologic area.


 What the Numbers Mean

 Each hydrologic unit is assigned an 8-digit attribute code that uniquely identifies each of the four levels of classification within four two-digit
 fields. An example is shown below using hydrologic unit code 04030203:

Level Code Description
Region 04 Great Lakes
Subregion 0403 Northwestern Lake Michigan
Accounting Unit 040302 Fox River, Wisconsin
Cataloging Unit
Lake Winnebago, Wisconsin

 A complete list of Hydrologic Unit codes, descriptions, names, and drainage areas can be found in the United States Geological Survey Water-Supply Paper 2294, entitled "Hydrologic Unit Maps".


 Below is a list of maps that will show you what the 2,4,6, and 8-digit HUCs look like on a national scale.

 HUC 2 map
 HUC 4 map
 HUC 6 map
 HUC 8 map




More sites with information about HUCs:

 EPA's Surf Your Watershed - A watershed in Surf Your Watershed is the 8-digit cataloging unit; gives info about watershed too

Map a watershed

Watershed Information Network (EPA)

 1:2,000,000-Scale Hydrologic Unit Maps

USGS Water Resources Division - Hydrologic Unit maps and spatial data

USGS National Hydrography Dataset

USGS GIS Data for Water Resources

EPA Office of Water

USGS Geographic Names Information System (GNIS)  - here you can find the location of a lake, have it mapped, and through a link to the EPA Surf your watershed site, find out what HUC it is in.  A great tool; we use it all the time.

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URL: https://nas.er.usgs.gov
Page Contact Information: Pam Fuller - NAS Program (pfuller@usgs.gov)
Page Last Modified: Wednesday, May 23, 2018


The data represented on this site vary in accuracy, scale, completeness, extent of coverage and origin. It is the user's responsibility to use these data consistent with their intended purpose and within stated limitations. We highly recommend reviewing metadata files prior to interpreting these data.

Citation information: U.S. Geological Survey. [2018]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [8/15/2018].

Contact us if you are using data from this site for a publication to make sure the data are being used appropriately and for potential co-authorship if warranted. For queries involving fish, please contact Pam Fuller. For queries involving invertebrates, contact Amy Benson.