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This U.S. Geological Survey map depicts a classification system that divides and subdivides the United States into successively smaller river basin units. These levels of subdivision, used for collection and organization of water resources data, are called hydrologic units. The hydrologic units outlined on the map represent natural and man made stream-drainage areas. The numeric identifiers associated with these units are hydrologic unit codes. These codes are used as administrative addresses of basins, and they do not necessarily represent natural watersheds. Detailed drainage, waterbodies, and names are also shown for the entire map area.
The map size is approximately 40x57 inches. For ordering information or assistance, phone 1-888-ASK-USGS.
Regions are the largest drainage basins delineated on the map. Subregions divide the regions, and they include the area drained by a river system, a closed basin(s), or a group of streams that form a coastal drainage area. Accounting units subdivide many of the subregions. Cataloging units are the smallest hydrologic subdivisions shown.
The hydrologic unit code is an eight-digit number that identifies each hydrologic unit. The code uniquely identifies each of the four levels-regional, subregional, accounting, and cataloging-of hydrologic classification within four two-digit fields.
Information on the National Atlas, access to this data, availability of other products and those of other Federal agencies in the National Atlas is given at: http://www.nationalatlas.gov/
U.S. Department of the Interior |
Information Sheet February 1999 |
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