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
||Average: 177,560 square miles
||Average: 16,800 square miles
||Basin (previously called Accounting Unit)
||Average: 10,596 square miles
||Sub-Basin (previously called Cataloguing Unit, EPA calls Watershed)
||Average: 703 square miles
||63-391 square miles
||16-63 square miles
Region: 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
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