There was a small town where the Musselshell river enters the Missouri river. It is now under water. I believe this is one of the river boat stops on the upper Missouri when on their way up to Fort Benton.
That's old fort musselshell, I got some stories in one of my books on it. Some good info on the site in "we pointed them north," by Teddy Blue Abbot. Besides being an old trading Post, in later years rustlers operated from there. Stuart's stanglers caught a few there. It is gone bank eroded and fell into the river.