 <unk> eastwards through the plains of eastern Montana , the Missouri receives the <unk> River from the north before crossing into North Dakota where the Yellowstone River , its greatest tributary by volume , joins from the southwest . At the confluence , the Yellowstone is actually the larger river . The Missouri then meanders east past <unk> and into Lake <unk> , the reservoir formed by Garrison Dam . Below the dam the Missouri receives the Knife River from the west and flows south to Bismarck , the capital of North Dakota , where the Heart River joins from the west . It slows into the Lake Oahe reservoir just before the <unk> River confluence . While it continues south , eventually reaching Oahe Dam in South Dakota , the Grand , Moreau and Cheyenne Rivers all join the Missouri from the west . 
