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 hydrololgic units with an average size of 226 square miles (585 sq. km.).