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OJll14 The Missouri River Basin mcludes the drainage area of the Missouri River and its tributaries. and encompasses one.sixth of the contiguous United States as shown in figure I. It includes all of Nebraska, most of Montana, North ,"d South Dakota, and Wyoming; about h,lf of Kansas and Missouri; smalkr parts of Colorado. Iowa, and Minnesota; and some territory in Canada. The basin is bounded sharply by tht.' Rocky Mountains on the west; ruughly by the Canadian Border on the north: by distinct to imperceptible drainage divides of the Souris- Red RIVer basins 011 the northeast; by the upper Mississippi River Basin on the east; and by the Arkansas- White-Red River basins on the south. Aside from international water accountability, thiS framework study is concerned only with that part of the basin within the United States. The Missouri River drains a watershed of 5] 3,3001 squ3re miles within the United States and about 9,700 sqwHe miles north of the International Boundary, which the river itself does not cross. Figure I shows the geogr3phic relation of the Missouri River Basin and its States to the Nation, and the primary drainage system WIthin the basin. Formed by the junction of the Jefferson, Gallatin. and Madison flvers at Three Forks, Mont.. the Missouri River tlows generally southeasterly 2,315 miles to its junction WIth the Mississippi River about IS miles above S1. Louis, Mo. The Yellowstone, Little Missouri. Cheyenne, Niobrara, Platte, Kansas, Osage, and Gasconade rivers. in downstream order, are the major tributary streams from the south and west; and the Milk, James, Big Sioux, Little Sioux, Grand, Jnd Chari ton rivers from the north and e::JSt. PHYSICAL CHARACTERISTICS There are three major physiographic divisions within the MIssouri Basin - the Interior HIghlands. the Interior Plains. and the Rocky Mountain System - as delineated in figure 2. Within the basin the Rocky Mountain System division includes parts of the Northern Rocky I Excludes 6,340 square miles in the Great Divide (Red Desert) Basin and other noncontributing drainage areas near Ihe basin boundary that were not made a part of this study. CHAPTER 2 DESCRIPTION OF THE BASIN Mountains, Middle Rocky Mountains. Wyoming Basin. and Southern Rocky Mountains provinces. The Interior Plains division includes parts of the Great Plains and Centra] Lowlands provinces. Sections Jnd subsectiuns within the Great Plains province include such distinct topographic features as the Black Hills in South Dakota and Wyoming, and the Sand Hills in Nebraska. That portion of the Interior Highlands division within the b<lsin is characterized by its Ozark Plateaus province. The Rocky Mountain System forms the western boundary of the basin and renects an exceptionally rugged topography, with many peaks surpassing 14,000 feet in elevation. The approximately 55,000-square.mile mountainous area is punctuated with milny high valleys. but the peaks and mountain spurs dominate the physical features. Extending eastw:Hd from the Rocky Mountain System division is the Interior Plains division which ch::Jracterizes the major portion of the Missouri Basin. The Great Plains province. within the division, is a 360.000.square-mile area Ihat forms the heartland of the basin. The eastern boundary of this province lies approximately along the 1500-foot contour, and the western boundary at the foot of the Rocky Mountain System averages about 5,500 feet in elevation. Average slopes west-to.east are about 10 feet to the mile. South and west of the Missouri River the surface mantle and topography have been developed largely by erosion of a fluvial plain extending from the mountains. The alluvial outwash laid down a heterogeneous mixture of mantle material. Simultaneous and subsequent water and wind erosion of the mantle produced a variable topographic relief. dependent on variations in climate and erodibility of the mantle. Within the Great Plains provmce are isolated mountainous areas developed by erosion of dome-type uplifts. Principal among these are the Black Hills in western South Dakota and northeastern Wyoming, an elliptical-shaped area 60 miles wide and 125 miles long. That portion of the Great Plains province north and east of the Missouri River, and at places extending south of the river. has been influenced by contmell\a\ glaciation. Here the topography was shaped mainly by erosion of the glacial drift and till. Morainic drift belts are in evidence and large boulders