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<br />
<br />CHAPTER 3
<br />DESCRIPTION OF THE MISSOURI RIVER BASIN
<br />
<br />Physical Characteristics
<br />
<br />The Missouri River Basin encompasses one-sixth of the
<br />contiguous area of the United States. It is bounded by the
<br />Rocky Mountains sharply on the west; by the Canadian Border
<br />roughly on the north, by the drainage divides of 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 ba-
<br />sins on the south. Figure 3 shows that the basin is divided into
<br />eight subbasins; the Upper Missouri, Yellowstone, Western
<br />Dakotas, Eastern Dakotas, Platte-Niobrara, Middle Missouri,
<br />Kansas, and Lower Missouri. The Basin includes all of the
<br />State of Nebraska; most of Montana, North Dakota, South
<br />Dakota, and Wyoming; about half of Kansas and Missouri;
<br />smaller parts of Colorado, Iowa, and Minnesota; and parts of
<br />southern Alberta and Saskatchewan provinces in Canada. The
<br />Missouri River and its tributaries drain 328.5 million acres
<br />within the United States and about 6.2 million acres within
<br />Canada, an area which is not covered in this report. Formed by
<br />the junction of the Jefferson, Gallatin, and Madison Rivers in
<br />southwestern Montana, the Missouri River flows generally
<br />southeastward 2,315 miles to its junction with the Mississippi
<br />River 15 miles above St. Louis, Missouri.
<br />
<br />The three major physiographic divisions within the
<br />Basin are shown in Figure 1: the Interior Highlands, the Interior
<br />Plains, and the Rocky Mountain System. The western bound-
<br />ary of the Basin is formed by the Rocky Mountain System, an
<br />area of exceptionally rugged topography, with many peaks sur-
<br />passing 14,000 feet (mean sea level) in elevation. This 35.2
<br />million-acre mountainous area is punctuated with many high
<br />valleys, but the peaks and mountain spurs domina\e the physi-
<br />cal features.
<br />
<br />Extending eastward from the Rocky Mountain System
<br />are the Interior Plains, some 230.4 million acres of which are
<br />classified as the Great Plains province and 56.3 million acres of
<br />which are called the Central Lowlands province. The Great
<br />Plains province is in the west-central part of the Basin and, in
<br />general, has a flat to gently rolling topography. Average west-
<br />to-east slopes are about 10 feet to the mile from an average
<br />elevation of 5,500 feet along the western boundary ot the Great
<br />Plains at the foot of the Rocky Mountains to the 1 ,500-foot con-
<br />tour that approximates the eastern boundary of the province.
<br />
<br />There are two other types of landfonns within the Great
<br />Plains. Isolated dome-type uplifts have formed rugged, almost
<br />mountainous areas such as the Black Hills of western South
<br />Dakota and northeastern Wyoming. The other type of landform
<br />consists of moderately sloping sand dunes stabilized by grass-
<br />es with many shallow basins, ponds, and swamps. Principal
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<br />among these is the Sand Hills area of north-central Nebraska.
<br />
<br />The other province within the Interior Piains, the Central
<br />Lowlands, extends eastward trom a line between Jamestown in
<br />east-central North Dakota and Salina in central Kansas to the
<br />Mississippi River drainage divide. The land is generally level
<br />except where stream development has created a hilly topog-
<br />raphy.
<br />
<br />In the southeastern part of the Basin is a 7 -million-acre
<br />area ot hilly to mountainous land called the Ozark Highlands.
<br />
<br />The climate within the basin is determined largely by the
<br />interaction of four extensive air masses: warm, moist air from
<br />the Gulf of Mexico; cool, moist air from the northern Pacific
<br />Ocean; cold, dry air from the northern polar regions; and hot,
<br />dry air from the plateaus in north-central Mexico. Because of
<br />the extreme differences in the characteristics of these four air
<br />masses, weather changes are sudden and severe. Local cli-
<br />mates can be greatly influenced by the Rocky Mountains, as
<br />evidenced by substantial temperature and precipitation
<br />anomalies. Orographic lilting of air masses can procuce pre-
<br />cipitation on the windward side of the mountains and dry, wann
<br />(chinook) winds on the leeward side.
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<br />15
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