The Bois de Sioux River (/bɔɪz də suː/) drains Lake Traverse, the southernmost body of water in the Hudson Bay watershed of North America. It is a tributary of the Red River of the North and defines part of the western border of the U.S. state of Minnesota, and the eastern borders of North Dakota and South Dakota. It is about 41 miles (66 km) in length. Bois de Sioux is a name derived from French meaning "Woods of the Sioux".