A watershed is a geographic area into which the surrounding waters, sediments, and dissolved materials drain, and whose boundaries extend along surrounding topographic ridges. Each watershed or unit has a unique hydrologic unit code (HUC). Hydrologic unit codes are a USGS cataloging system that arranges watersheds from the largest area or region (2 digits - 03) to the smallest (12 digits - 030502020101). SCDHEC divides South Carolina into 8 major river basins, which include the Savannah River Basin, the Saluda River Basin, the Edisto River Basin, the Salkehatchie River Basin, the Broad River Basin, the Catawba River Basin, the Santee River Basin, and the Pee Dee River Basin. These basins are further subdivided into 185 10-digit watersheds and 960 12-digit watersheds statewide.