 To the north , the much lower <unk> Divide separates the Missouri River watershed from those of the Oldman River , a tributary of the South Saskatchewan River , as well as the <unk> , <unk> , and smaller tributaries of the Red River of the North . All of these streams are part of Canada 's Nelson River drainage basin , which empties into Hudson Bay . There are also several large <unk> basins between the Missouri and Nelson watersheds in southern Alberta and Saskatchewan . The Minnesota and Des Moines Rivers , tributaries of the upper Mississippi , drain most of the area bordering the eastern side of the Missouri River basin . Finally , on the south , the <unk> Mountains and other low divides through central Missouri , Kansas and Colorado separate the Missouri watershed from those of the White River and Arkansas River , also tributaries of the Mississippi River . 
