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CUOl75 APPLICATION TO LOWER SOUTH PLATTE RIVER . For purposes of this study, a reach approximately 90 miles long at the lower end of the South Platte River in Colorado was chosen to be modeled. The reach is essentially that formerly known as Water District 64. General Description of Reach The study reach begins a few miles upstream from the gaging station at Balzac and ends at the Colorado-Nebraska State line (Julesburg gaging station). The reach contains about 120,000 acres of irrigated lands served by 30 ditch systems and 3 major reservoirs, In addition, there are about 750 irrigation wells, some of which serve the same land and are supplemental to the ditch-water supplies, An estimated 25,000 addi- tional acres are irrigated from ground water only. Stream-aquifer system . The water supply for the study reach comes from an hydraulically connected surface-water and ground-water system--generally referred to simply as a stream-aquifer system. The principal aquifer involved is the alluvium of the South Platte River from which most of the 750 irri- gation wells withdraw their supplies, The alluvium varies from 2-1/2 to 7-1/2 miles in width, averaging about 4.3 miles. The saturated thickness exceeds 100 feet under about 76,000 acres between the North Sterling Canal headgate and the State line. The alluvium contains an estimated 3.5 million acre-feet of ground water under about 388 square miles. The principal source of recharge to the alluvial aquifer is the deep percolation of irrigation water from canals, reservoirs and irrigated fields overlying the aquifer. In addition, other investigators have estimated that approximately 75,000 acre-feet of water a year flows into this reach of the South Platte alluvium from the High Plains ground-water system south of the river (Waltz and Sunada, 1972). The water added to the ground-water system in the study reach is generally sufficient to maintain a water-table level higher than the . -18-