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<br />I'lrl,[)"" <br />~J "'.1 _'. ~ '-- '1: <br /> <br />17 <br /> <br />}J <br /> <br />II. HYDROLOGICAL SETTING <br /> <br />The, South Platte water system in Colorado has evolved from the <br />"natural" state to one which is highly developed. The natural hydrology <br />is, in fact, dominated by man in the plains. It is managed by a multi- <br />tude of, institutions. A social-political-hydrological equilibrium <br />generally exists. This chapter describes some of the sal ient physical <br />features of this system (1). <br />Area, The South Platte River Basin has an area of 24,030 square <br />miles (6a,238 sq. km), with 19,020 square miles (49,262 sq. km) in <br />Colorado. Figure 2-1 shows the basin on a map in relation to the State <br />of Colora~o and also Wyoming and Nehraska. <br />Physiography. The western part of the basin is dominated by <br />mountains. known as the "Front Range," which are a part of the Colorado <br />Rocki es. Several peaks ri se above 14,000 feet (4267 m). <br /> <br />The mountain snowpack is, of course, the main source of water. The <br /> <br /> <br />tributary streams drain the snowpack, with drainage occurring mostly as <br /> <br /> <br />spring runoff. Then throughout the rest of the year groundwater derived <br /> <br />from the ;snowpack sustains a modest base flow. As the streams emerge <br />from their mountain canyons, canal diversions form a reverse dendritic <br />pa ttern carryi ng water to off-stream storage reservoirs and adjacent <br />irrigated lands. Figure 2-4 is a map of the basin showing the stream <br />system and irrigated lands. <br />Preci:pitation. The average October-April precipitation in the <br />Basin varies from 3.5 inches in the lower plains to 22.5 inches in the <br />mountains,: occurring mostly as snowfall. The average May-September <br />precipitation varies from 6.5 inches in the lower plains to 15.0 inches <br />in the mountains. <br /> <br />:1 <br />