It rises on the plains of northern Fergus County and flows eastward .
The Sacagawea River is a tributary of the Musselshell River , approximately 30 mi ( 48 km ) long , in north - central Montana in the United States .
Sacagawea River rises on the plains of northern Fergus County and flows eastward .
The Sacagawea River is a tributary of the Musselshell River , approximately 30 mi ( 48 km ) long , in north - central Montana in the United States .