A set of maps depicting approved boundaries of, and numerical codes, for, river-basin units of the Unites States has been developed by the U.S. Geological Survey. These "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. Geological Survey's National Water Data Network, and (3) he 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 square miles (1,813 square kilometers). 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, in indexing and inventorying hydrologic data and information, in cataloging water-data acquisition activities, and in a variety of other applications. Because the maps have undergone extensive review by all principal Federal, regional, and Stat water-resource 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 uses are given in the report. They hydrologic unit codes shown on the maps have been approved as a Federal Information Processing Standard for use by the Federal establishment.