 The Missouri makes a bend to the southeast as it winds through the Great Plains , receiving the Niobrara River and many smaller tributaries from the southwest . It then proceeds to form the boundary of South Dakota and Nebraska , then after being joined by the James River from the north , forms the Iowa – Nebraska boundary . At Sioux City the Big Sioux River comes in from the north . The Missouri flows south to the city of Omaha where it receives its longest tributary , the Platte River , from the west . <unk> , it begins to define the Nebraska – Missouri border , then flows between Missouri and Kansas . The Missouri swings east at Kansas City , where the Kansas River enters from the west , and so on into north @-@ central Missouri . To the east of the Kansas City , Missouri receives , on the left side , the Grand River . It passes south of Columbia and receives the Osage and <unk> Rivers from the south downstream of Jefferson City . The river then rounds the northern side of St. Louis to join the Mississippi River on the border between Missouri and Illinois . 
