Hydrologic Unit Codes (HUC) Metadata

The United States is divided and sub-divided into successively smaller hydrologic units which were classified into four levels: regions, sub-regions, accounting units and cataloging units based on 1:100,000-scale data. However, it became apparent over the last several years that the current 8-digit hydrologic unit maps were inadequate for many purposes. Because of this, multiple member agencies on the Federal Geographic Data Committee (FGDC), Subcommitte on Spatial Water Data have been working to establish a Federal interagency standard covering mapping and delineation of hydrologic units that would be suitable for all agencies. This current work will provide a hydrologically correct, seamless and consistent national Geographic Information System (GIS) database at a scale of 1:24,000, that has been extensively reviewed and matches the USGS topographical 7.5 minute quads. The new levels are called watershed (5th level, 10-digit) and subwatershed (6th level, 12-digit). The watershed level is typically 40,000 to 250,000 acres and subwatershed level is typically 10,000 to 40,000 acres with some as small as 3,000 acres. An estimated 22,000 watersheds and 160,000 subwatersheds will be mapped to the 5th and 6th level.

Hydrologic Unit Levels

Hydrologic Unit Level Name Digits Size Units
1 Region 2 Average: 177,560 square miles 21
2 Sub-Region 4 Average: 16,800 square miles 222
3 Basin (previously called Accounting Unit) 6 Average: 10,596 square miles 352
4 Sub-Basin (previously called Cataloguing Unit, EPA calls Watershed) 8 Average: 703 square miles 2,149
5 Watershed 10 63-391 square miles 22,000 (estimate)
6 Sub-Watershed 12 16-63 square miles 160,000 (estimate)

The first level of classification divides the Nation into 21 major geographic areas, or regions. These geographic areas 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, which includes a number of rivers draining into the Gulf of Mexico. Eighteen of the regions occupy the land area of the conterminous United States. Alaska is region 19.

SubRegion: The second level of classification divides the 21 regions into 222 subregions. A subregion includes the area drained by a river system, a reach of a river and its tributaries in that reach, a closed basin(s), or a group of streams forming a coastal drainage area.

Basin or Accounting Unit: The third level of classification subdivides many of the subregions into accounting units or basins. These 352 hydrologic accounting units nest within, or are equivalent to, the subregions.

SubBasin or Cataloguing Unit: The fourth level of classification is the cataloging unit or subbasin, Cataloging Units sometimes are called "watersheds." A cataloging unit is a geographic area representing part of all of a surface drainage basin, a combination of drainage basins, or a distinct hydrologic feature. These units subdivide the subregions and accounting units into smaller areas. There are 2150 Cataloging Units in the Nation.

Watershed: The fifth level of classification. These are delineated by taking into consideration the significant drainages within the 4th level cataloging units.

SubWatershed: The sixth level of classification are currently the smallest element in the hierarchy of hydrologic units. These are delineated using "natural" watersheds (those which discharge at a single point and includes all of the area above that point which contributes to the drainage discharge), size limitations, or physical locations such as dams or gaging stations.

Links of Interest

Page last updated on: January 15, 2008

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