National Water-Quality Assessment (NAWQA) Program - South Platte River Basin
The South Platte River Basin has a drainage area of about 24,300 mi2 (Dennehy, 1991) and is located in parts of three States (fig.1) - Colorado (79 percent of the basin), Nebraska (15 percent of the basin), and Wyoming (6 percent of the basin). The South Platte River originates in the mountains of central Colorado at the Continental Divide and flows about 450 mi northeast across the Great Plains to its confluence with the North Platte River at North Platte, Nebraska. Altitude in the basin ranges from 14,286 ft at Mt. Lincoln on the Continental Divide to 2,750 ft. at the confluence of the South Platte and North Platte Rivers. The basin includes two physiographic provinces - the Front Range Section of the Southern Rocky Mountain Province and the Colorado Piedmont Section of the Great Plains Province.
The basin has a continental-type climate modified by topography, in which there are large temperature ranges and irregular seasonal and annual precipitation. Mean temperatures increase from west to east and on the plains from north to south (Gaggiani and others, 1987). Areas along the Continental Divide average 30 in. or more of precipitation annually, which includes snowfall in excess of 300 in. In contrast, the annual precipitation on the plains east of Denver, Colorado, and in the South Park area in the southwest part of the basin, ranges from 7 to 15 in. (fig. 2). Most of the precipitation on the plains occurs as rain, which typically falls between April and September, whereas most of the precipitation in the mountains occurs as snow, which typically falls between October and March.
The three-State area of the South Platte River Basin has about 2.8 million people, over 95 percent of who live in Colorado. The basin contains the most concentrated population density in the Rocky Mountain region (fig. 3), located along the Front Range urban corridor in Colorado where the mountains meet the plains. Population densities outside the urban corridor are small and centered in small towns located along the principal streams. The principal economy in the mountainous headwaters is based on tourism and recreation; the economy in the urbanized south-central region mostly is related to manufacturing, service and trade industries, and government services; and the economy of the basin downstream from Denver is based on agriculture and livestock production.
Land use and land cover in the South Platte River Basin (fig. 4) during 1975-80 (Feagas, and others, 1983) is divided into: 41 percent rangeland, 37 percent agricultural land, 16 percent forest land, 3 percent urban or built-up land, and 3 percent other land. Rangeland is present across all areas of the basin except over the high mountain forests. Agricultural land is somewhat more restricted to the plains and the South Park area near Fairplay, Colo. Forest land occurs in a north-south band in the mountains. Urban or built-up land is present primarily in the Front Range urban corridor. The 'other land' category includes: water (110 mi2), barren lands (160 mi2), tundra (400 mi2), and perennial snow and ice (1 mi2). Barren lands primarily are areas under construction or are areas of strip mining, quarries, or gravel pits.
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