Colorado Water Resources Circular No. 20

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0751 Canon City, the river crosses alluvial deposits and is more tranquil in nature. Truck farming predominates on irrigated land just east of Canon City. Wheat, corn, hay, alfalfa, and sugar beets are also grown in this reach. The natural streamflow of the upper Arkansas River results from snowmelt from the high mountain peaks bordering the basin, rainfall, and return flow from irrigated land. This flow, however, is supple- mented by eight transmountain diversions including the new Charles H. Boustead Tunnel. Average flow in 1962 -71 for the other seven diversions was 71,370 acre -feet per year. The Boustead Tunnel, which began divert- ing in 1972, is expected to bring an average of 69,200 acre -feet per year to the Arkansas River basin as part of the Fryingpan- Arkansas Project. Average flow at 13 current streamflow gaging stations on the mainstem Arkansas River is given in table 1. Three reservoirs on tributaries of the upper Arkansas River- - Turquoise Lake, Twin Lakes Reservoir, and Clear Creek Reservoir - -store water for release during times of critical irrigation and industrial demand. The current combined usable storage of these reservoirs is 186,400 acre -feet. The Fryingpan- Arkansas-Project will increase this storage by 363,700 acre -feet (including 234,000 acre -feet of storage in Pueblo Reservoir on the Arkansas River mainstem west of Pueblo). River hydrographs during a reservoir release are shown in figure 2. • As table 1 indicates, the upper Arkansas River basin has an area of over 5,000 square miles. It includes all of Lake, Chaffee, Fremont, and Custer Counties, and parts of Saguache, Park, Teller, El Paso, and Pueblo Counties. Precipitation in the basin ranges from less than 10 to more than 40 inches per year, generally increasing with elevation. On an annual basis, and assuming no change in storage, about 86 percent (4 million acre -feet) of the water entering the basin is consumed by evapotranspiration and about 14 percent leaves the basin as surface - and ground -water outflow (P. A. Emery, written commun., 1972). TRAVEL TIME OF RESERVOIR RELEASES Records of 51 reservoir releases made during the period 1939 -71 were analyzed to show the relation of release travel time to river flow. Figure 3 shows this relationship. It indicates that time of travel of reservoir releases varies from 30 to 70 hours depending on the ante- cedent river flow, the travel time being longer for lower river flows. The time scale shows the number of hours elapsed between the re- lease from Twin Lakes Reservoir and the arrival at the Colorado Canal headgates. The time of release was determined either directly from chart records for the outlet of Twin Lakes, or by subtracting about 111 hours from the time the release arrived at the Granite gaging station (07086000). The time of arrival at the Colorado Canal headgates was 0 5