High Quality Content by WIKIPEDIA articles! The Sacagawea River is a tributary of the Musselshell River, approximately 30 mi (48 km) long, in north-central Montana in the United States. It rises on the plains of northern Fergus County and flows eastward. The river formerly joined the Musselshell five miles above the confluence of the Musselshell with the Missouri, but it now flows into the arm of Fort Peck Lake on the Missouri formed by the mouth of the Musselshell.