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I 9 o I I I I 9 I ! I I I I I I I I m 15 16 III. HYDROLOGY Cherry Creek, the Cherry Creek Dam was completed 1n 1953. Thls dam and reservoir controls the runoff from the 386 square miles of high plains catchment. The total storage capacity 1n the lake behind the dam 1s 228,400 acre.feet. The flood contro1 storage 1s 70.960 acre~feet. the multi. purpose storage is 13,950 acre-feet and the surcharge storage 1s 134,500 acre.feet (Corps of Engineers, 1974). Water was first stored 1n tile lake 1n1957. Basin Description .Climate. Upstream from Metropolitan Denver, the South Platte River basin consists of two units. Major streams to the west and south- west have headwaters on the Continental Divide 1n the Rocky Mountain Range. Strean~ to the south and east have headwaters on the high plains or foot- hills. This diversity of topography results in two entirely different runoff regimes. At the summits of the mountains, the mean annual temperature averages Jess than 32 degrees F. The precipitation depends largely on elevation and exposure to moisture-bearing winds. On the Continental Divide, winter snow is usually substantially greater than summer rains. The a~nual total precipitation is almost double that experienced on the plalns (Corps of Engineers, 1977). Runoff from the mountainous region comes primarily from snowmelt in the months of May and June. The high plains region has a continental climate with light precipi- tation and large ranges in temperature. The annual mean temperature is approximately 50 degrees F. The range in extremes is from 40 degrees below zero to 100 degrees above. Approximately 70 percent of the annual.pr~cipi- tation falls in the six.month period April to September, most of It 1n d~wnpours frc~ su~r thunderstorms (Corps of Engineers, 1~77). May is . the wettest month. Winter snowfall averages from two to SlX feet dependlng on the location. The long.term average annual precipitation at Denver 1s 14 inches. The runoff fnom the South Platte River 8asin is modified by diversions to and from the river system and by stor<lge in reservoirs. The numerous diversions from the South Platte River system in and upstream from the study reach is beyond the scope of this study. Dams and Reservoirs. There are four major dams On the South Platte River system upstream from Metropolitan Denver. The l~cations of the dams are shown in Fig. 3 and the pertinent data are listed 1n Table 4. The three older dams, Cheesman, Antero and Eleven-Mile Ca~on are owned by the City of Denver. The storage provided by these dams is for municipal and industrial water supply. Any flood storage is incidental. The newest dam is Chatfield ff1Jl1E!dfately upstream from Metropolitan Denver. Chatfield was built by the Corps of Engineers solely to provide flood control for the Metro area. No\',' a low pool level is maintained to provide a lake for recreational use. The total stora9~ capacity of Chatfield take is 385,000 acre-feet. Of this value, 215.000 acre.feet of capacity is for flood storage and 20,000 acre-feet is forsed1ment control (Corps of Engineers, 1974. 1977). The dam was completed in 1975 and contro1s the runoff from 3,018 square miles of mostly mountainous catchment. The Corps of Engineers has constructed two other flood control dams in Metropolitan Denver. Approximately 12 miles upstream from the mouth of Completed in 1979, The Bear Creek Dam is located approximately six miles upstream from the confluence of Bear Creek with the South Platte River. The dam provides 26,000 acre.feet of flood storage and will reduce the peak of the lOO-year flood from 30,000 cubic feet per second to 1000 cubic feet persecortd(GineryAssociates, Inc., 1979). The total storagebehindthe dam is 52.000 acre-feet. Water was first stored in 1980. The Corps of Engineers (~JM-Phl1lips-Reister, Inc., 1974) has estimated that the loo.year flood in the Denver reach of the South Platte River with the three storage dams in operation is 15.000 cubic feet per sec- ond. The channel of the river cart handle approximately 16,000 cubic fee t per second without appreciable flooding damage. The numerous small dams constructed for irrigation and local munici- pal water supply, soil conservation or local flood control do not appreciably affect the flood flow in the South Platte River. imports. Water is imported from the Colorado River Ilasin to Metropol tan Denvermainlythrou9h the Moffat Tunnel and the Harold D. Roberts Tunnel. Both are owned by the City of Denver. Comp1eted in 1936, the Moffat Tunnel carries water diverted from the Williams Fork of the Frasier River over to South Boulder Creek. From there. it is rerouted to the Denver Water Board's distribution system. The Roberts Tunnel carries water from Dillon Reservoir on the Blue River through the Continental Divide to the North Fork of the South Platte River. The City of Aurora imports water from the Eagle River on the Western Slope and from the Arkansas River on the Eastern Slope through the Homestake Pipeline, The diversion from the Eagle River began in 1967 and from the Arkansasinlg7J. Inlg75,theannual import rates for the Moffat and Roberts Tunnels and the Hcmestake Pipeline were 59,320, 28.6S0 and 6450 acre.feet. These three facilities accounted for 98.8 percent of the total annual import of water to the South Platte River System at Metropolitan Denver (Corps of Engineers, 1977). All imported water is diverted out of the South Plijtte River prior to pij55in~ the Denv~r streamgage at Ninete~nth Stre~t in downtown Denv~r. How. ever, a portiOn of the imported water returns to the river system either as flow from the urban sewage system. surface runoff or groundwater seepage. The return flow for all municipal and industrial water in the Metropolitan Denver is estimated to be 75 percent (Corps of Engineers. 1977). Most or all of the return flow which reaches the South Platte River will have done