river, central North Dakota, U.S. It rises in Sheridan county and flows east past the Spirit Lake Sioux reservation, then south through Valley City, near which Baldhill Dam impounds Lake Ashtabula, and near Lisbon it turns northeast to join the Red River of the North about 10 miles (16 km) north of Fargo. The Sheyenne River is about 325 miles (525 km) long, the longest river located entirely within North Dakota. It was named for the Cheyenne Indians. Sheyenne National Grassland, near Lisbon, preserves one of the state's last remaining areas of tallgrass prairie.