The Moreau River, a tributary of the Missouri River, runs through the Cheyenne River Reservation, creating steep cut banks along the way. Home to the Cheyenne River Lakota Nation, the reservation covers 4,267 square miles (11,051 square kilometers) in the state of South Dakota and is the fourth largest reservation in the United States. The Missouri River forms the reservation’s eastern boundary and the Cheyenne River forms its southern boundary.