The Watershed Boundary Dataset contains boundaries of “hydrologic units” throughout the country. According to the USDA , “Hydrologic unit boundaries define the aerial extent of surface water drainage to a point.” The hydrologic units are nested in a hierarchical system, decreasing in size from basins (HUC 6), to subbasins (HUC 8), to watersheds (HUC10), to subwatersheds (HUC 12). The boundaries can assist in environmental, land use, and water use planning.