 Flowing eastwards through the plains of eastern Montana , the Missouri receives the Poplar 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 Williston and into Lake Sakakawea , 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 Cannonball 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 . 
 Flowing eastwards through the plains of eastern Montana , the Missouri <a href='uninflect' title='1'><mark>receives</mark></a> the Poplar River from the north before crossing into North Dakota where the Yellowstone River , its greatest tributary by volume , <a href='uninflect' title='2'><mark>joins</mark></a> from the southwest . At the confluence , the Yellowstone <a href='drop_aux' title='3'><mark>is</mark></a>actually the larger river . The Missouri then <a href='uninflect' title='4'><mark>meanders</mark></a> east past Williston and into Lake Sakakawea , the reservoir formed by Garrison Dam . Below the dam the Missouri <a href='uninflect' title='5'><mark>receives</mark></a> the Knife River from the west and <a href='uninflect' title='6'><mark>flows</mark></a> south to Bismarck , the capital of North Dakota , where the Heart River <a href='uninflect' title='7'><mark>joins</mark></a> from the west . It <a href='uninflect' title='8'><mark>slows</mark></a> into the Lake Oahe reservoir just before the Cannonball River confluence . While it <a href='uninflect' title='9'><mark>continues</mark></a> south , eventually reaching Oahe Dam in South Dakota , the Grand , Moreau and Cheyenne Rivers all join the Missouri from the west . 
Flowin eastwards through the plains of eastern Montana , da Missouri receive the Poplar River from the north before crossing into North Dakota where tdaYellowstone River , its greatest tributary by volume , join from thdaouthwest . At thedanfluence , the Yellowstone actually the dager river . The Mdaouri then meander east past Williston and into Lake Sakakawea , the redavoir formed by Garrison Dam . Below the dam the Missouri receive the Knife River from the wesdand flow south to Bismarck , the capital of North Dakota , where the Heardaiver join from the west dat slow into the Lake Oahe reservoir jus 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 Missoudafrom the west .