Course and Characteristics

The Missouri flows northward through the Rockies, passes the Gates of the Mountains gorge, and breaks upon the Great Plains between Helena and Great Falls, Montana. It veers north and east across northern Montana, turns south and east, crosses the Dakotas, and forms the boundary between Nebraska and Kansas on its right bank and South Dakota, Iowa, and Missouri on its left. Turning eastward at Kansas City, the river crosses Missouri and empties into the Mississippi.

Except in the Rockies, where its headwaters are swift mountain streams, the Missouri is normally a shallow, slowly moving river. Its average drop is only about 1 1/2 feet per mile (28 cm per km). Like the Mississippi, the Missouri varies enormously in flow. Near its mouth, it has carried as much as 800,000 cubic feet (22,650 m 3 ) of water per second and as little as 13,000 cubic feet (370 m 3 ) during periods of drought.

Low water occurs in winter, when much of the surface water in the basin is frozen. High water (March through June) comes with spring rains and thaws and the melting of snow in the mountains. As a result, the river rises and falls as much as 35 feet (11 m) and occasionally changes its channel. When low, it often becomes choked with sand and mud.

Many rivers drain into the Missouri. From west to east, they include the Marias, Musselshell, Milk, Yellowstone, Little Missouri, Cheyenne, Niobrara, James, Big Sioux, Platte, Kansas, Chariton, and Osage.