The river formerly joined the Musselshell five miles above the confluence of the Musselshell with the Missouri .
River now flows into the arm of Fort Peck Lake on the Missouri formed by the mouth of the Musselshell .
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 .