Flowing eastwards through the plains of eastern Montana , the Missouri receive the Poplar River from the north before crossing into North Dakota where the Yellowstone River , its greatest tributary by volume , join from the southwest . At the confluence , the Yellowstone is actually the larger river . The Missouri then meander east past Williston and into Lake Sakakawea , the reservoir formed by Garrison Dam . Below the dam the Missouri receive the Knife River from the west and flow south to Bismarck , the capital of North Dakota , where the Heart River join from the west . It slow into the Lake Oahe reservoir just before the Cannonball River confluence . While it continue south , eventually reaching Oahe Dam in South Dakota , the Grand , Moreau and Cheyenne Rivers all join the Missouri from the west .