MISSOURI RIVER. The principal affluent of the Mississippi and the longest river of the United States (Map: United States, G 3). It is formed in southwestern Montana by the con fluence of the Jefferson, Madison, and Gallatin rivers. The longest branch, the Jefferson, has its farthest source'in Red Rock Creek, which rises on the slopes of the Red Rock range of the Rocky Mountains, in Madison County. Montana, a few miles from the headwaters of the North Fork of the Snake. The middle and largest branch, the Madison, rises in the Yellowstone Park near the source of the Yellowstone River. From the junction of the three forks, the Missouri flows north and east across Montana into North Da kota, where it describes a long curve toward the southeast and then crosses the whole width of South Dakota. After reaching the Nebraska boundary it divides this State from Iowa and Mis souri. forms the northeastern boundary of Kansas, and finally takes an easterly course across Mis souri, joining the Mississippi 20 miles above Saint Louis. Its length to the source of the Jefferson is about 2950 miles, and to the source of the Madi son about 2910 miles. With the lower Mississippi, the river has a total length of about 4200 miles, which is equaled by no other river in the world. The Missouri is a swift and turbid stream, navi gable only by flat-bottomed steamboats. Dur ing the flood period in early summer it can be ascended to Great Falls, Mont., about 2300 miles
from the mouth, hut in low water navigation is suspended above the junction of the Yellowstone. At Great Falls the Missouri passes over a series of cataracts, descending a vertical distance of 350 feet in 16 miles. The highest falls are S7 feet. About 145 miles above this point is the Gate of the Rocky Mountains, a narrow rock gorge 12 miles in length, whose perpendicular walls rise 1200 feet above the river. The lower course of the Missouri. lying within the great plains, has been graded so as to offer no inter ruptions. in this part it is often a mile or more wide. The chief tributaries are the Milk and Yellowstone, in Montana: Little Missouri, in North Dakota; Cheyenne, James, White, and Big Sioux, in South Dakota; :Niobrara and Platte, in Nebraska; Kansas and Osage. in Kan sas and Missouri. It drains the greater part of the territory between the Mississippi and the summits of the Rocky Mountains. The Arkansas and Red rivers are the only other large streams in this region that contribute their waters di rectly to the Mississippi. The area of the basin exceeds 500.000 square mild. A number of thriv ing cities are located on the Missouri, including Kansas City, Leavenworth, Atehison, Omaha, Sioux City, Pierre, Bismarck, and Great Falls, the last being the centre of a great copper smelting industry which utilizes the power of the Falls.