Missouri River ('Big Muddy'), the principal branch of the Mississippi River, is formed by the confluence of the Jefferson, Gallatin, and Madison rivers, at Gallatin City, Montana, 4132 feet above the sea-level. These rivers rise in the Rocky Mountains, close to the sources of the Columbia and Colorado rivers, and to the Continental Divide. The Madison has the remotest source in a small lake of the same name in Yellowstone National Park in Wyoming, 44° 19' N. lat. and 110° 5C W. long., at an elevation of 7632 feet, and flows north-west and north to the junction of the Three Forks. Thence the Missouri flows N. and NE. to Fort Benton, the head of navigation, 225 miles from Gallatin City. About 40 miles above Fort Benton are the Great Falls, where the river descends 327 feet in 15 miles by a series of cataracts (the highest 87 feet). From Fort Benton the course is easterly, the river being flanked by bluffs about a mile apart until it passes the rapids 400 miles below, when the valley opens to a width of 10 miles. The Milk River is its first large tributary, but at the boundary of North Dakota the still larger Yellowstone (1152 miles) joins it. From the last junction, which is the head of navigation in the low-water season, the Missouri flows E. and SE. through North Dakota, to Bismarck (1610 feet above sea-level, where it is crossed by the splendid bridge of the Northern Pacific Railroad), and through South Dakota to Sioux City, whence flowing south the river bounds Nebraska and Kansas on the right and Iowa and Missouri on the left. On receiving the tributary Kansas the stream turns to the east, and flowing across the state of Missouri pours its muddy waters into the Mississippi, 20 miles above St Louis. The Missouri is 3047 miles long, of which 2682 are called navigable, but owing to its tortuous, treacherous, and obstructed channel navigation is attended with great risks. The chief towns on the banks are Bismarck, Yankton, Sioux City, Omaha, Council Bluffs, Nebraska City, St Joseph. Atchison, Leavenworth, and Kansas City.

See Commerce and Navigation of the Mississippi (1888), and other works by Humphrey and Abbot (1861), and Glazier (1893).