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MISSOURI RIVER, the principal western tributary of the Mississippi river, U.S.A. It is formed at Gallatin City, in the Rocky Mountain region of south-western Montana, by the confluence of the Jefferson, Madison and Gallatin forks; thence it flows N. into the plains, which it traverses in a course at first N.E., then E. Entering North Dakota, the river turns gradually to the S.E., then S., and again S.E., 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 20 m. above St Louis, and 2547 m. below the confluence of the three forks. The stream which is known as the Jefferson Fork 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 Missouri; it rises on the border between Montana and Idaho, 20 m. west of the western boundary of the Yellowstone National Park, near the crest of the Rocky Mountains, 8000 ft. above the sea, and 398 m. beyond Gallatin City; and with this and the Lower Mississippi the Missouri forms a river channel 4221 m. in length, the longest in the world. The Madison and Gallatin forks rise within the Yellowstone Park, where the former is fed by geysers and hot springs and the latter by both hot springs and melting snow. The Yellowstone river, which is the principal tributary of the Missouri, traverses the park. The Missouri drains a basin having an area of about 580,000 sq. m.; this includes the eastern slope of the Rocky Mountains from the northern border of the United States to the middle of Colorado, and its larger tributaries take their rise in those mountains. 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 Niobrara 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 deposits of silt 50 to 100 ft. or more in depth. 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, the trees fall in and are carried down stream as snags (or “sawyers”), which are especially dangerous to navigation. The variation of level is great and it varies greatly in different parts of the river's course: it is about 19 ft. at Kansas City, about 25 ft. at St Charles, Missouri, and about 8 ft. at Fort Benton, Montana. It is estimated that the Missouri's average discharge per second amounts to about 94,000 cub. ft., and that each year it carries into the Mississippi 550,000 tons of silt. 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 snow on the mountains, and the river is navigable from early spring to midsummer as far as Fort Benton, within 40 m. of the Great Falls and 2285 m. above the mouth. Above Great Falls the river is navigable to Three Forks.

The mouth of the Missouri was discovered in 1673 by Marquette 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 dépôt; but the first exploration of the river from its mouth to its headwaters was made in 1804-1805 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 (made from the log of a cottonwood tree); the pirogue (usually two canoes side by side and with a floor over them on which to place the cargo); the bullboat (made by covering a framework of willow poles with the hides of bison bulls); the mackinaw boat (made, of boards and having a flat bottom); and the keelboat (a vessel of some pretensions, with a keel from bow to stern, 60 to 70 ft. in length, with a breadth of beam from IS to 18 ft., and drawing 20 to 30 in. of water). A canoe, pirogue, bullboat, or mackinaw boat was propelled by two or more men with paddles, poles, or oars; but to propel a keelboat up the river required 20 to 40 men who walked along the shore and pulled a corvelle, a line about 1000 ft. long and fastened to the mast. An average of about 15 m. a day was made with a keelboat going up the river. The first attempt to navigate the Missouri with steamboats was made in the spring of 1819, when the “Independence” made a trip from St Louis to the mouth of the Chariton river and back. 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 Northwest. The traffic was at its height in 1858, when no fewer than 60 regular packets were engaged in it, but its decline began in the following year with the completion of the Hannibal & St Joseph railway to St Joseph, Missouri, and 20 years later it had nearly disappeared. In an attempt to regulate railway rates, however, four boats were run between Kansas City and St Louis between 1890 and 1894 by the Kansas City & Missouri Transportation Company, and in 1906 the Missouri River Valley Improvement Association was formed at Kansas City. Congress began to make appropriation for the removal of snags about 1838, and forty years later appropriations were begun for a general improvement which in 1884 was placed under the charge of the Missouri River Commission. In 1890 its work was restricted to that part of the river below Sioux City and in 1902 the Commission was abolished. Up to the 30th of June 1908 the Federal government had expended $11,398,881 for the improvement of the river.

See H. M. Chittenden, History of Early Navigation on the Missouri

River (New York, 1903); P. E. Chappel, A History of the Missouri

River (Kansas City, 1905); J. V. Brower, The Missouri River and

its Utmost Source (St Paul, 1896); J. M. Hanson, The Conquest of

the Missouri (New York, 1909); L. M. Jones, “The Improvement of the Missouri River and its Usefulness as a Traffic Route,” in Annals of the American Academy of Political and Social Science (Jan. 1908), and the Annual Reports of the Chief of Engineers,

U.S. Army.