Hydrologic Unit Information

Hydrologic Units

The Water Resources Council developed a hierarchial classification of hydrologic drainage basins in the United States. Each hydrologic unit is identified by a unique hydrologic unit code (HUC) consisting of two to eight digits based on the four levels of classification in the hydrologic unit system (see HUCS.TXT below). The first level of classification consists of 21 water-resources regions as shown below. Subsequent levels include 222 sub-regions, 352 accounting units, and 2150 cataloging units.


The gray lines are state lines, the blue lines are major rivers, and the white lines are water-resources region boundary lines.

Hydrologic Unit Information Files

HUCS.TXT gives an explanation of hydrologic units, regions, sub-regions,
accounting units, cataloging units, and hydrologic unit codes (HUC).

HUC_NAME.TXT gives the full description of all the hydrologic units.

See the HUC_NAME.FMT file for details of the file layout.

HUC_AREA.DAT gives the drainage area of the hydrologic units by region,
subregion, accounting unit, and cataloging unit.

See the HUC_AREA.FMT file for details of the file layout.

SREGIONS.DAT gives the names of the hydrologic regions and subregions.

See the SREGIONS.FMT file for details of the file layout.

HUC 1:250k provides digital data on hydrologic unit boundaries for the United States.

For a comprehensive discussion of hydrologic units, see:

Seaber, P.R., Kapinos, F.P., and Knapp, G.L., 1987, Hydrologic units maps: U.S. Geological Survey Water-Supply Paper 2294, 63 p.

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Last modified: Tue Apr 21 11:00:13 EDT 1998