MISSOURI RIVER, the principal western tributary of the Mississippi river, U.S.A. It is formed at Three Forks, in the Rocky Mountain region of south-western Montana, by the con fluence of the Jefferson, Madison and Gallatin, thence it flows north into the plains, which it traverses in a course at first north east, then east. Entering North Dakota, the river turns gradu ally to the south-east, then south, and again south-east, traversing both North and South Dakota. It forms the eastern boundary of Nebraska and, in part, of Kansas, and crosses Missouri in an easterly course to its junction with the Mississippi 17 m. above St. Louis, and 2,551 m. below the confluence of the three forks. The stream which is known as the Jefferson river in its lower course, Beaver Head river in its middle course and Red Rock creek in its upper course, is really the upper section of the Mis souri; it rises on the border between Montana and Idaho, 20 m. W. of the western boundary of the Yellowstone National park, near the crest of the Rocky mountains, 8,000 ft. above the sea, and 394 m. beyond Three Forks, and with this and the Lower Mississippi the Missouri forms a river channel about 4,240 m. in length, the longest in the world. The Madison and Gallatin forks rise within the Yellowstone park, and the Yellowstone river, which is the principal tributary of the Missouri, traverses the park. The Missouri drains a basin having an area of about 580,00o sq.m., 2,55o sq.m. of which are in Canada.
Besides the Yellowstone and the three forks, there are the Platte, which rises in two large branches in Colorado, and the Milk, which rises in north-western Montana. The Kansas in Kansas, the James and Big Sioux in the Dakotas, and the Nio brara in Nebraska, are the principal tributaries wholly of the plains. In the mountain region the Missouri flows through deep canyons and over several cascades. Below Great falls the slower current is unable to carry all the silt brought down from the mountains and plains, and consequently a winding and unstable channel has been formed on deep deposits of silt. Bends in the river continue to develop by erosion until the neck between two of them is cut off, and in the process numerous islands, sand bars and crescent-shaped lakes are formed. Cottonwood, willow, cedar and walnut trees grow upon the banks that are for a time left undisturbed, but years later the eroding current returns to undermine these banks and the trees fall in and are carried down stream as snags, which are specially dangerous to navigation. The
minimum discharge is 12,000 cu.ft. per sec. at Sioux City and 23,000 at Kansas City. The maximum discharge is about 200, 00o cu.ft. per sec. at Sioux City and approximately 500,000 at Kansas City. The waters of the Missouri begin to rise in March, and a high water stage is reached in April, as a result of the spring rains and the melting snow on the plains; a second high stage is produced in June by the melting of the snow on the mountains. The fluctuation between extreme high and low water is 13.3 ft. at Pierre; 22.5 ft. at Sioux City; and at Kansas City 34•6 feet. The river is navigable to Fort Benton 2,285 m. above the mouth.
The mouth of the Missouri was discovered in 1673 by Mar quette and Joliet, while they were coming down the Mississippi. Early in the 18th century French fur-traders began to ascend the river, and in 1764 St. Louis was established as a depot ; but the first exploration of the river, from its mouth to its headwaters, was made in 18o4–o5 by Meriwether Lewis and William Clark. Until many years later the commerce on the river was restricted to the fur trade, and was carried on with such primitive craft as the canoe, the pirogue, the bullboat, the mackinaw boat and the keelboat.
The first attempt to navigate the Missouri with steamboats was made in 1819. The American Fur company began to use steamers in 1830, and from then until the advent of railways the steamboat on the Missouri was one of the most important factors in the development of the North-west. The traffic was at its height in 2858, when no fewer than 6o regular packets were engaged in it, but its decline began in the following year with the completion of the Hannibal and St. Joseph railway to St. Joseph, Mo., and later on it had almost entirely disappeared. Various attempts have been made to revive river traffic in an effort to secure cheaper transportation for the valley.
The U.S. Congress has authorized the improvement of the Missouri for navigation, with a view to securing a navigable channel 6 ft. deep from the mouth to Sioux City, and for snag ging, with some rock removal and bank protection to Fort Ben ton. The Government has spent about $26,000,000 on the section below Kansas City, about $3,000,000 on the section between Kansas City and Sioux City, and about $3,500,000 on the por tion of the river above Sioux City. It is expected that a channel capable of being used for barge traffic will be available to Kansas City in 1930.