Regions are the largest watersheds shown. Regions
contain either the drainage areas of a major river, such as the
Missouri region, or the combined drainage areas of several rivers,
such as the Texas-Gulf region. They were designated
by the U.S. Water Resources Council and are used for comprehensive
planning. Regions are differentiated by color in the National Atlas
Map Maker. The regional hydrologic unit name is indicated on the
Hydrologic Units Wall Map.
Subregions divide the regions and they include the area drained
by a river system, a section of a river and its tributaries in
that reach, a closed basin(s), or a group of streams forming a
coastal drainage area.
Accounting units subdivide many of the subregions. They are used
by the USGS for managing national water data.
Cataloging units are the smallest hydrologic subdivisions shown.
Most are larger than 700 square miles in area. A cataloging unit
is a geographic area representing part or all of a surface drainage
basin, a combination of drainage basins, or a distinct hydrologic
Regional and subregional boundaries serve as accounting and cataloging
A hydrologic unit code is an eight-digit number that identifies
each of the watersheds into which the country has been divided
for the purpose of water-resources planning and data management.
The code uniquely identifies each of the four levels of hydrologic
classification within four two-digit fields.
|Accounting unit code
|Cataloging unit code
The first two digits identify the water-resources region; the
first four digits identify the subregion; the first six digits
identify the accounting unit; and the addition of two more digits
identifies the cataloging unit.
On the National Atlas Hydrologic Units Wall Map, the code is
shown as a group of six digits and a separate group of two digits.
This example shows how to identify, by hydrologic unit code, the
sample cataloging unit shown below.