The Missouri make a bend to the southeast as it wind through the Great Plains , receiving the Niobrara River and many smaller tributaries from the southwest . It then proceed to form the boundary of South Dakota and Nebraska , then after being joined by the James River from the north , form the Iowa – Nebraska boundary . At Sioux City the Big Sioux River come in from the north . The Missouri flow south to the city of Omaha where it receive its longest tributary , the Platte River , from the west . Downstream , it begin to define the Nebraska – Missouri border , then flow between Missouri and Kansas . The Missouri swing 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 receive , on the left side , the Grand River . It pass south of Columbia and receive the Osage and Gasconade Rivers from the south downstream of Jefferson City . The river then round the northern side of St. Louis to join the Mississippi River on the border between Missouri and Illinois .