A set of maps depicting approved boundaries of, and numerical codes for, river-basin units of the United States has been developed by the U.S. Geological Survey. These "State Hydrologic Unit Maps" are four-color maps that present information on drainage, culture, hydrography, and hydrologic boundaries and codes of: (1) the 21 major water-resources regions and the 222 subregions designated by the U.S. Water Resources Council; (2) the 352 accounting units of the U.S. Geological Survey's National Water Data Network; and (3) the 2,149 cataloging units of the U.S. Geological Survey's Catalog of Information on Water Data. The maps are plotted on the Geological Survey State base-map series at a scale of 1:500,000 and, except for Alaska, depict hydrologic unit boundaries for all drainage basins greater than 700 mi2 (1,813 km^). A complete list of all the hydrologic units, along with their drainage areas, their names, and the names of the States or outlying areas in which they reside, is contained in the report. These maps and associated codes provide a standardized base for use by water-resources organizations in locating, storing, retrieving, and exchanging hydrologic data; indexing and inventorying of hydrologic data and information; cataloging of water-data acquisition activities; and a variety of other applications. Because the maps have undergone extensive review by all principal Federal, regional, and State water-resources agencies, they are widely accepted for use in planning and describing water-use and related land-use activities, and in geographically organizing hydrologic data. Examples of these usages are given within the report. The Hydrologic Unit Codes shown on the maps have been approved as a Federal Information Processing Standard for use by the Federal establishment. This report describes the U.S. Geological Survey's standard map series called "State Hydrologic Unit Maps" and presents the codes, names, and boundaries of hydro!ogic units in the United States and the Caribbean outlying areas. The four-color maps depict a hydro!ogic system that divides the United States into 21 major regions. These regions are currently (1984) further subdivided into 222 subregions, 352 accounting units, and, finally, into 2,149 cataloging units^These four levels of subdivisions, used for the collection and organization of hydro!ogic data, are called "hydro!ogic units." The identifying numeric codes associated with these units are "hydro!ogic unit codes." All hydro!ogic units have been assigned names that usually correspond to the principal hydro!ogic feature within the unit. The State Hydrologic Unit Maps show drainage, hydrography, culture, and political and hydrologic unit boundaries and codes, thus providing a standard geographic and hydrologic framework for detailed water-and related land-resource planning. Also, included on the maps are the Federal Information Processing Standards State and county codes (U.S. National Bureau of Standards, 1983). Recognizing that such maps were needed by almost everyone working in wat...
