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.

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 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	04030204	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".
 

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 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

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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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