Grand River (South Dakota)
The Grand River is a tributary of the Missouri River in North Dakota and South Dakota in the United States.
It is formed by the confluence of the North Fork (which rises in North Dakota) and South Fork (which rises in South Dakota) in northwestern South Dakota near Shadehill in Perkins County, near several parcels of the Grand River National Grassland. It flows east, through the Standing Rock Indian Reservation and joins the Missouri in Lake Oahe, approximately 10 mi (16 km) northwest of Mobridge. The lower 20 mi (32 km) of the river form an arm of the Lake Oahe reservoir.