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<br />" <br /> <br />2002 <br /> <br />General basin characteristics <br /> <br />. <br /> <br />The 5,200 mi2 upper Arkansas River basin includes the 140 mile river reach from the headwaters near Lead. <br />ville to Portland (Figure 1). The Sawatch and Sangre de Cristo Ranges fDlm the southern and western boundaries. <br />and the Mosquito Range fonns the nonhern boundary of the study area. Altitudes within the study area range from <br />14,433 ft at Mount Eiben. the highest peak in Colorado, to about 5.030 fl at Portland. The mountain ranges are <br />composed of sedimentary rocks of Paleozoic age. and igneous and metamorphic rocks principally of Precambrian <br />age. There is a transition from igneous and metamorphic rocks to sedimentary rocks between Park-dale and Port- <br /> <br />land (Figure 1). <br /> <br />Figure 1 near here. <br /> <br />. <br /> <br />Precipitation and tra/lsmountain diversions are the principal sources of water in the basin. The average <br /> <br />annual precipitation ranges from more than 40 inches to less than 10 inches. generally decreasing with decreasing <br />altitude. Precipitation in the mountains produces a deep snowpack during the winter months; snowmelt supplies a <br />large percentage of the total annual streamflow. Transmountain diversions of water from the Colorado River basin <br />are supplied from several sources high in the basin. Much of this water is routed to Twin Lakes Reservoir (Figure <br /> <br /> <br />I) via closed conduit and open channel flow (Abbott. 1985). The transmountain water is stored in Twin Lakes Res. <br /> <br /> <br />ervoir until being released to meet downstream irrigation and municipal-supply demands. During the study period, <br /> <br /> <br />April 1 990.March 1993. the mean monthly discharge of the Arkansas River ranged from 70 ft3/s at Leadville to <br /> <br />688 ft3/s at Parkdale. The largest source of tributary inflow to the river is water released from Twin Lakes Reser- <br /> <br />voir into Lake Creek. The mean monthly reservoir release from Twin Lakes Reservoir to Lake Creek and the <br /> <br /> <br />Arkansas River was 284 ft3fs. Reservoir releases represented about 50 percent of the annual flow in the Arkansas <br /> <br /> <br />River at Granite. just downstream from the confluence of Lake Creek and the river. and about 30 percent of the <br /> <br /> <br />total annual flow at Portland, at the downstream end of the basin. The other tributaries in the basin contribute sub. <br /> <br /> <br />. stantially less flow. <br /> <br />4 <br />