Big Sioux River

The Big Sioux River is a tributary of the Missouri River, 295 mi (470 km) long, in eastern South Dakota and northwestern Iowa in the United States. The United States Board on Geographic Names settled on "Big Sioux River" as the stream's name in 1961.

The Big Sioux River rises in Roberts County, South Dakota on the Coteau des Prairies and flows generally southwardly through Grant, Codington, Hamlin, Brookings, Moody, and Minnehaha Counties, past the communities of Watertown, Castlewood, Bruce, Flandreau, Egan, Trent, Dell Rapids, and Baltic to Sioux Falls, where it passes over a waterfall which gives that city its name. Downstream of Sioux Falls and the community of Brandon, the Big Sioux is used to define the boundary between South Dakota and Iowa, flowing along the eastern borders of Lincoln and Union Counties in South Dakota, and the western borders of Lyon, Sioux and Plymouth Counties in Iowa, past the communities of Canton, Fairview, Hudson, North Sioux City, and Dakota Dunes in South Dakota and Hawarden, Beloit, and Akron in Iowa. It joins the Missouri River from the north at Sioux City, Iowa.