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Hydrologic Units


  Hydrologic Units


  Shaded relief map of the United States showing watershed boundaries
View watershed boundaries.
  The United States Geological Survey (USGS) monitors the quantity and quality of surface and ground waters throughout the nation. The USGS organizes watersheds or drainage basins of the United States into a hydrologic system that divides and subdivides the United States into successively smaller watersheds. These levels of subdivision, used for organization of hydrologic data, are called "hydrologic units." The hydrologic units outlined in the National Atlas represent natural and manmade watersheds. The Hydrologic Units Wall Map also includes the identifying numerical codes called "hydrologic unit codes," associated with these units. The hydrologic unit codes describe the relation of the hydrologic units to each other to represent the way smaller watersheds drain areas that together form larger watersheds. The hydrologic unit names and codes are obtained on the National Atlas Map Maker by use of the "Identify" tool.
  Sample map of Hydrologic UnitsSample of "identify" results
Sample of Hydrologic Units from the Map Maker with names and codes resulting from using "identify" at a point in eastern Wisconsin.

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 feature.

Regional and subregional boundaries serve as accounting and cataloging unit boundaries.

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.

For example:
Regional code 07
Subregional code 0707
Accounting unit code 070700
Cataloging unit code 07070003

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.

  Graphic of watershed boundaries and codes

Watershed boundaries and codes as shown on the Hydrologic Units Wall Map.
  The boundaries and numerical codes were modified from the State Hydrologic Unit Maps (1:500,000 scale) prepared by the USGS in cooperation with the U.S. Water Resources Council. Descriptions, names, and drainage areas of hydrologic units were published in "Hydrologic Unit Maps" by Seaber, Kapinos, and Knapp, U.S. Geological Survey Water-Supply Paper 2294, 1987.
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