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.

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.).

The 6-Digit Hydrologic Units, a component of the Watershed Boundary Dataset (WBD), depict the third level hydrologic subdivision of the United States. A 6-digit hydrologic unit may subdivide a 4-digit hydrologic unit or may be equivalent. There are 401 6-digit hydrologic units with an average size of 10,486 square miles (27,159 sq. km.).

The 8-Digit Hydrologic Units, a component of the Watershed Boundary Dataset (WBD), depict the fourth level hydrologic subdivision of the United States. A hydrologic unit can accept surface water from upstream drainage areas and may define a drainage area with single or multiple outlet points. An 8-digit hydrologic unit, or sub-basin, is a common management unit for water resource planning and analysis. There are 2,362 8-digit hydrologic units with an average size of 1,783 square miles (4,619 sq. km.).

The 10-Digit Hydrologic Units, a component of the Watershed Boundary Dataset (WBD), depict the fifth level hydrologic subdivision of the United States. A hydrologic unit can accept surface water from upstream drainage areas and may define a drainage area with single or multiple outlet points. A 10-digit hydrologic unit, or watershed, divides sub-basins into smaller management areas. There are 18,679 10-digit hydrologic units with an average size of 226 square miles (585 sq. km.).

The 12-Digit Hydrologic Units, a component of the Watershed Boundary Dataset (WBD), depict the sixth level hydrologic subdivision of the United States. A hydrologic unit can accept surface water from upstream drainage areas and may define a drainage area with single or multiple outlet points. A 12-digit hydrologic unit, or subwatershed, supports water resource management and planning activities at a localized and site-specific level. There are 101,779 12-digit hydrologic units with an average size of 41 square miles (107 sq. km.).