Missouri River (i. e., Mud river), the principal tributary of the Mississippi. It properly forms one stream with that river, being much greater in length and volume than the other branch which bears that name above the mouth of the Missouri. It rises near the boundary between Montana and Idaho, among the Rocky mountains, in several small streams, the principal of which are Jefferson and Wisdom rivers (the latter rising within a mile of the head springs of Clarke's fork of the Columbia), whose sources lie between lat. 44° 20' and 45° 35' N., and Ion. 112° and 114° W., uniting about lat. 45° 15', Ion. 110° 30'. According to some geographers, the Missouri properly begins about 80 m. further E., where the stream formed by the Jefferson and Wisdom, which on this hypothesis retains thus far the former name, is joined by the Madison and Gallatin. The Madison, the middle and largest fork, by some considered the true source, rises in the National Park in N. W. Wyoming, near the sources of the Snake and Yellowstone. After a devious course N. from the junction of the three forks to about lat. 48°, the Missouri runs E. through Montana into Dakota, where it is joined by (lesser) White Earth river.
Its general direction is S. E. thence to the Mississippi, which it joins in lat. 38° 50' 50" N., Ion. 90° 14' 45" W., after separating Nebraska from Iowa, forming a small part of the dividing line between Missouri and Kansas and Nebraska, and flowing across the whole state of Missouri. Its length to the Madison fork source is 2,908 m., which added to 1,286 m., the length of the lower Mississippi, makes its whole course to the gulf 4,194 m. It has commonly been navigated as far as the mouth of the Yellowstone, on the border of Dakota and Montana, but it may be ascended by steamboats much further, to the Great falls almost at the very base of the mountains, and about 2,500 m. from the Mississippi. There is no serious obstruction to navigation below this point, though at certain seasons the water is shallow, owing to its passing through a dry and open country in its upper course, and being subject to extensive evaporation. It is generally turbid and rapid. In its lower course it is bordered by a narrow alluvial valley of great fertility, back of which lie generally extensive prairies. At its mouth it is over half a mile wide, and in many places? it is much wider.
Its principal tributaries are the Yellowstone, Little Missouri, Big Cheyenne, (greater) White Earth, Niobrarah, Platte or Nebraska, Kansas, and Osage on the right, and the Milk, Dakota, Big Sioux, Little Sioux, and Grand on the left. It will thus be seen that the Missouri receives till the great rivers which rise on the eastern declivity of the Rocky mountains, with the single exception of the Arkansas, and a large share of the waters which lie between its own bed and that of the Upper Mississippi. The area which it drains is estimated at 518,000 sq. rn. The most important places on its hanks are Fort Benton in Montana, Yankton in Dakota, Sioux City and Council Bluffs in Iowa, Omaha in Nebraska, Atchison and Leavenworth in Kansas, and St. Kansas City, Lexington, Booneville, Jefferson City, and St. Charles in Missouri. About 400 m. from its source the river passes through a narrow gorge denominated the "Gates of the Rocky mountains." It is 5 3/4 m. long, and the perpendicular walls of rock, which rise directly from the water to the height of 1,200 ft., are only 450 ft. apart. For the tirst 3 in. there is hut one spot where a foothold could be obtained between the water and the rock. The Great falls are 145 m. below this point.