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<br />oonS87 <br /> <br />contiguous area of the United States. It is bounded by the Rocky <br />M::luntains on the west, by Canada on the north, by the SOuris-Red <br />River.basins on the northeast, by the upper Mississippi River <br />_ basin on the east, and by the Arkansas-White-Red River basins on <br />the south. Figure 2 shows that the basin includes all of the <br />State of Nebraska: most of M::lntana, SOuth Dakota, and wyoming: <br />about half of Kansas, Missouri and North Dakota: smaller parts of <br />Colorado, Iowa, and Minnesota: and parts of southern Alberta and <br />Saskatchewan in Canada. The Missouri River is formed by the <br />junction of the Jefferson, Gallatin, and Madison Rivers in <br />southwestern Montana and flows generally southeastward 2,315 <br />miles to its junction-with the Mississippi River 15 miles above <br />St. Louis, Missouri. The Missouri River and its tributaries <br />drain 328.5 million acres within the United States and about 6.2 <br />million acres within Canada. The Canadian drainage area is not <br />considered in this report. <br /> <br />Three major physiographic divisions within the basin, as <br />shown in figure 2, are the Interior Highlands, the Interior <br />Plains, and the Rocky Mountain Systelll. The western boundary of <br />the basin is formed by the Rocky M::luntain System, an area of <br />exceptionally rugged topography. Many of its peaks surpass <br />14,000 feet (mean sea level) in elevation. This 35.2 million <br />acre mountainous area is marked by many high valleys, but the <br />peaks and mountain spurs are predominant. <br /> <br />Extending eastward from the Rocky Mountain System are the <br />Interior Plains, including some 230.4 million acres classified as <br />the Great Plains province and 56.3 million acres called the <br />Central Lowlands province. The Great Plains province is in the <br />west central part of the basin and has, in general, a flat to <br />gently rolling topography. Average west to east slopes are about <br />10 feet to the mile from an average elevation of 5,500 feet along <br />the western boundary of the Great Plains at the foot of the Rocky <br />Mountains to the 1,500-foot contour that approximates the eastern <br />boundary of the province. <br /> <br />There are two other types of landforms within the Great <br />Plains. Isolated dome-type uplifts have formed rugged, almost <br />mountainous areas such as the Black Hills of western South Dakota <br />and northeastern wyoming. The other type of landform consists of <br />moderately sloping sand dunes stabilized by grasses with many <br />shallow basins, ponds, and marshes. Principal among these is the <br />Sand Hills area of north central Nebraska. <br /> <br />-13- <br /> <br />