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 hydrololgic units with an average size of 1,783 square miles (4,619 sq. km.).