Hydrologic Unit Maps

A set of four-color "Hydrologic Unit Maps" show the approved boundries of, and numerical codes for river-basin units of the United States. These maps, developed by the U.S. Geological Survey, present information on drainage, culture, hydrography, and hydrologic boundaries. Plotted at a scale of 1:500,000 units, the Geological Survey State base-map are a complete list of all the hydrologic units, along with their drainage area names and the States in which they reside. These maps and associated codes provide a standardized base for useage by water-resouce organizations, in storing, retrieving, and exchanging of hydrologic data.