Big Sioux River (SOO), 295 mi/470 km long, S.Dak. and Iowa; rises in E Day Co., near Waubay, in NE S.Dak.; flows S past Watertown and Sioux Falls; receives Pipestone Creek from NE, then forms Iowa–S.Dak. state line (last 85 mi/137 km), continuing S past Canton (S.Dak.) and Akron (Iowa), to join the Missouri R. at Sioux City (Iowa). Passes through an agr. region that produces corn, soybeans, oats; hogs, beef cattle.
