how much of South Dakota is west of the longest river in North America?
West River (South Dakota) West River is the portion of the state of South Dakota located west of the Missouri River with well over one-half of the land area and between one-quarter and one-third of the population of the state.
North Fork Musselshell River The North Fork Musselshell River is a tributary of the Musselshell River, approximately 35 miles (56 km) long, in Montana in the United States.