The Musselshell River is a tributary of the Missouri River, 341.9 miles long from its origins at the confluence of its North and South Forks near Martinsdale, Montana to its mouth on the Missouri River. It is located east of the Continental divide entirely within Montana in the United States. Counting its pre-confluence tributaries, it measures 425–500 miles in …The Musselshell River is a tributary of the Missouri River, 341.9 miles long from its origins at the confluence of its North and South Forks near Martinsdale, Montana to its mouth on the Missouri River. It is located east of the Continental divide entirely within Montana in the United States. Counting its pre-confluence tributaries, it measures 425–500 miles in length.