Water resource region

A water resource region is the first level of classification used by the United States Geological Survey to divide and sub-divide the United States into successively smaller hydrologic units as part of the U.S. hydrologic unit system.

This first level of classification divides the United States into 21 major geographic areas, or regions. These geographic areas contain either the drainage area of a major river, or the combined drainage areas of a series of rivers.