The 295 mile long Big Sioux River (SOO), rises near the Grant/Day County lines southeast of Waubay, and flows mostly south past Watertown and Pelican Lake, then past Castlewood and Bruce, near Brookings and Sioux Falls. There it receives Pipestone Creek from Minnesota, then forms the Iowa-South Dakota state line, continuing south past Canton, and Akron (Iowa), to join the Missouri River at Sioux City (Iowa).
The river nutures an agricultural region that produces corn, soybeans, oats, hogs, and beef cattle.
