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<br />I <br /> <br />U',., r .,... <br />:Ud,j~'~o <br /> <br />I' <br /> <br />smaller parts of Colorado, Iowa, and Minnesota; and parts of southern Alberta <br /> <br />I <br /> <br />and Saskatchewan in Canada. The Missouri River is formed by the junction of <br /> <br />the Jefferson, Gallatin, and Madison Rivers in southwestern Montana and flows <br /> <br />I <br /> <br />generally southeastward 2,315 miles to its junction with the Mississippi River <br /> <br />15 miles above St. Louis, Missouri. The Missouri River and its tributaries <br /> <br />1 <br /> <br />drain 328.5 million acres within the united States and about 6.2 million acres <br /> <br />I <br /> <br />within Canada. The Canadian drainage area is not considered in this report. <br /> <br />Three major physiographic divisions within the basin, as shown in figure <br /> <br />1 <br /> <br />2-1, are the Interior Highlands, the Interior Plains, and the Rocky Mountain <br /> <br />System. The western boundary of the basin is formed by the Rocky Mountain <br /> <br />I <br /> <br />System, an area of exceptionally rugged topography. Many of its peaks surpass <br /> <br />I <br /> <br />14,000 feet (mean sea level) in elevation. This 35.2-million-acre mountainous <br /> <br />area is marked by many high valleys, but the peaks and mountain spurs are <br /> <br />I <br /> <br />predominant. <br /> <br />Extending eastward from the Rocky Mountain System are the Interior Plains, <br /> <br />I <br /> <br />including some 230.4 million acres classified as the Great plains province and <br /> <br />56.3 million acres called the Central Lowlands province. The Great Plains <br /> <br />I <br /> <br />province is in the west-central part of the basin and has, in general, a flat <br /> <br />I <br />I <br /> <br />to gently rolling topography. Average west-to-east slopes are about 10 feet to <br /> <br />the mile fram an average elevation of 5,500 feet along the western boundary of <br /> <br />the Great Plains at the foot of the Rocky Mountains to the 1,500-foot contour <br /> <br />that approximates the eastern boundary of the province. <br /> <br />I <br /> <br />There are two other types of landforms within the Great Plains. Isolated <br /> <br />I <br /> <br />dome-type uplifts have formed rugged, almost mountainous areas such as the <br /> <br />Black Hills of western South Dakota and northeastern Wyoming. The other type <br /> <br />I <br /> <br />of landform consists of moderately sloping sand dunes stabilized by grasses <br /> <br />with many shallow basins, ponds, and swamps. Principal among these is the Sand <br /> <br />I <br /> <br />Hills area of north-central Nebraska. <br /> <br />I <br /> <br />2-3 <br />