The Broad River/Beaufort River/Port Royal Sound Basin (hydrologic unit 03050208) is located in Allendale, Hampton, Jasper, and Beaufort Counties, and encompasses 6 watersheds and 934 square miles. The Broad River/Beaufort River/Port Royal Sound Basin flows through the Coastal Zone region. Of the 597,659 acres, 32.5% is forested land, 20.7% is agricultural land, 19.4% is forested wetland (swamp), 10.7% is water, 10.2% is nonforested wetland (marsh), 6.4% is urban land, and 0.1% is barren land. The urban land percentage is comprised chiefly of a portion of Hilton Head Island and the Beaufort area. There are approximately 1,482 stream miles, 1,129 acres of lake waters, and 54,485 acres of estuarine areas in this basin.
The Coosawhatchie River originates near the Town of Allendale, and accepts drainage from Black Creek (Lake George Warren) and Cypress Creek before merging with the Pocotaligo River to form the Broad River. The Broad River joins the Chechessee River and the Beaufort River to form Port Royal Sound.