Water Quality Assessment of the South Platte River Basin, Colorado , Nebraska, and Wyoming

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`alluvial aquifer' or `alluvium'); the unconsolidated High Plains aquifer in eastern Colorado, southeastern Wyoming, and western Nebraska; and the consolidated rocks of the Denver Basin aquifer system in the Front Range urban corridor area. The Denver Basin aquifer system includes four named units, which, from strati - graphicaly lowest to highest are: the Laramie -Fox Hills, Arapahoe, Denver, and Dawson aquifers. The unconfined alluvial aquifer is hydraulically connected with the South Platte River along its main stem and major perennial tributaries and is recharged by precip- itation, by leakage from streams, reservoirs, and ditches, and by percolation of applied irrigation water. The High Plains aquifer generally is unconfined, but is confined locally by lenses of silt and clay. The Denver Basin aquifer system consists of consolidated rocks, which underlie the South Platte River Basin; this aqui- fer system is recharged in outcrop areas by rainfall, snowmelt, and streamflow. Discharge from all three primary aquifers occurs through wells, seeps, springs, streams, and evapotranspiration. The natural hydrology of the South Platte River Basin has been altered considerably by water develop- ment. The quantity of water in the basin has been increased an average of 400,000 acre -ft/yr through interbasin transfers of water from the Colorado, Arkan- sas, and North Platte River Basins (Litke and Appel, 1989). Occurrence of water has been modified in both space and time through a complex network of ditches and reservoirs (fig. 8 and table 1); the South Platte River is one of the most regulated rivers in the United States. Flows in the river during fall and winter (low flow) since the early 1900's have been maintained pri- marily by ground -water return flows from agricultural lands (Boyd, 1897; Huff and others, 1975, p. 17), whereas flows during spring and summer (high flow) are dominated by a snowmelt runoff that is attenuated by reservoir storage and irrigation diversions. An examination of flow conditions during water year 1991 gives a general picture of water routing along the South Platte River, although flows during the 1991 water year were about 25 percent below average. The South Platte River upstream from Denver is regu- lated by water - supply reservoirs, and most of the water is diverted via pipelines to water - treatment plants; the mean flow downstream from these diversions at South Platte River at Waterton (fig. 1) was 75 ft3 /s. The river next flows into Chatfield Reservoir, which is a flood - control reservoir whose releases generally match inflows; mean flow downstream from the dam was 74 ft3 /s. In the Chatfield to Henderson reach of the river, water is added by two major tributaries (Bear Creek, mean flow of 51 ft3 /s; Clear Creek, mean flow of 84 ft3 /s) and by wastewater - treatment -plant discharges (8 plants contribute about 275 ft3 /s), but more than 100 ft3 /s is removed by the Burlington Ditch and other irrigation ditches for irrigation downstream from this reach; mean flow downstream from Denver at South Platte River at Henderson was 393 ft3 /s. In the Henderson to Kersey reach of the river, three major tributaries enter the river: the St. Wain River (mean flow of 208 f0/s), the Big Thompson River (mean flow of 64 ft3 /s), and the Cache la Poudre River (mean flow of 119 ft3 /s); mean flow at South Platte River at Kersey was 808 ft3 /s. Although the inputs to this reach (total of 780 ft3 /s) are about equal to the flow at Kersey, this balance masks the process of irrigation diversions and irrigation return flows (primarily ground -water return flows); several hundred cubic feet per second of water are diverted for irrigation through this reach. Flows on the South Platte River reach their maximum at Kersey. There are no major tributaries to the South Platte down- stream from Kersey; the large prairie drainages (Bijou Creek, Kiowa Creek, Lodgepole Creek) flow only dur- ing large rainfall events or from irrigation return flows. From Kersey to North Platte, water is primarily removed by irrigation diversions and added by ground- water return flows from irrigation. At several loca- tions, almost the entire river flow can be diverted. Dur- ing 1991, mean flow was 470 ft3 /s at Weldona, 365 ft3A at Julesburg, 311 ft3A at Roscoe, Nebraska, and 216 ft3 /s at North Platte, Nebraska. Water Use The estimated total offstream water use in the South Platte River Basin in 1990 was 3,900 MgaUd (4.4 million acre -ft). Of this amount, 71 percent is sur- face water, and 29 percent is ground water. Ground water is withdrawn primarily from the alluvial aquifer. The principal uses were irrigation (70.8 percent), power generation (14.6 percent), and domestic use (8.4 percent). Large instream uses of water include hydroelectric use (1,600 MgaUd) and reservoir evapo- ration (220 MgaUd). Water use is discussed in detail by Dennehy and others (1993). Irrigation is the largest water use throughout the basin, except in the lower South Platte River in Nebraska where power generation is the largest use. Most of the domestic water use in the basin is present along the Front Range urban corridor; about 40 percent of this domestic use is for lawn watering (Litke and Appel, 1989). The remaining five water -use categories DESCRIPTION OF THE SOUTH PLATTE RIVER BASIN 11