The 2-Digit Hydrologic Units, a component of the Watershed Boundary Dataset (WBD), depict the major water resource regions of the United States. 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. This first level of classification divides the United States into 22 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.
Watershed Boundary Dataset, 4-Digit Hydrologic Units
The 4-Digit Hydrologic Units, a component of the Watershed Boundary Dataset (WBD), depict the second level hydrologic subdivision of the United States. A 4-digit hydrologic unit includes the area drained by a river system, a reach of a river and its tributaries in that reach, a closed basin(s), or a group of streams forming a coastal drainage area. There are 240 4-digit hydrologic units with an average size of 17,521 square miles (45,378 sq. km.).