 Immediately before the <unk> Ice Age , the Missouri River was likely split into three segments : an upper portion that drained northwards into Hudson Bay , and middle and lower sections that flowed eastward down the regional slope . As the Earth plunged into the Ice Age , a pre @-@ <unk> ( or possibly the <unk> ) <unk> diverted the Missouri River <unk> towards its present confluence with the Mississippi and caused it to integrate into a single river system that cuts across the regional slope . In western Montana , the Missouri River is thought to have once flowed north then east around the Bear <unk> Mountains . <unk> are found in some spots along the river in western Montana . <unk> of the continental ice sheets diverted the river and its tributaries , causing them to pool up into large temporary lakes such as <unk> Lakes Great Falls , <unk> and others . As the lakes rose , the water in them often spilled across adjacent local drainage divides , creating now @-@ abandoned channels and <unk> including the <unk> <unk> , 100 miles ( 160 km ) long . When the glaciers retreated , the Missouri flowed in a new course along the south side of the <unk> , and the lower part of the Milk River tributary took over the original main channel . 
