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As part of a national investigation of trap efficiency of detention reservoirs, a study was made of fluvial sediment in the drainage area of K-79 Reservoir in north-central El Paso County, Oolo. This study by the U.S. Geological Survey was begun in 1956 and terminated in 1965. During this period, the average annual precipitation was near normal, although annual precipitation ranged from much below normal to much above normal. Drainage from most of the area passed through one or more upstream structures before reaching K-79 Reservoir. From August 29, 1956, when the original survey of the reservoir was made, to July 27, 1965, when the final resurvey was made, observed sediment discharge from the reservoir was about 2,610 tons, or 2.5 acre-feet; sediment deposition in the reservoir was about 7.5 acre-feet. Trap efficiency was 75 percent as computed from computed volume of sediment outflow and measured volume of sediment desposition. Trap efficiency was 83 percent computed from the observed weight of sediment outflow and the computed weight of sediment deposition; the few available data on volume weight and particle sizes of deposited sediment suggest a volume weight of 80 pounds per cubic foot, or about 13,000 tons of sediment deposited in the reservoir. Most of the sediment that entered K-79 Reservoir during 1956-60 originated in the downstream one-third of the drainage area, and during 1961-65 originated in a small area downstream from hydraulic structures Jtmilt during 1960. All the bedload discharge and most of the suspended-sediment discharge from a large part of the drainage area were trapped before reaching K-79 Reservoir. Flocculation of sediment within the reservoir probably is not a significant cause of deposition in the reservoir. The runoff from the area is of low mineralization and is not conducive to rapid flocculation, and the detention time for runoff is only a few hours. Neither the amount of sediment deposition in, nor the trap efficiency of, K-79 Reservoir should be regarded necessarily as representative of similar but unstructured areas in the Colorado Piedmont or High Plains. Under present structured conditions in the K-79 drainage area, the effective life of the reservoir should be exceptionally long. Dl D2 SEDIMENTATION IN SMALL DRAINAGE BASINS INTRODUCTION A study of the characteristics of fluvial sediment in the drainage area of K-79 Eeservoir, which is on Kiowa Creek in north-central El Paso County, Colo., began in April 1956 and ended in September 1965. This study was part of a national investigation of trap efficiency of detention reservoirs and was made by the Water Resources Division of the Geological Survey, U.S. Department of the Interior, in cooperation with the Soil Conservation Service, U.S. Department of Agriculture. The main purpose of the study was to determine the quantity and characteristics of sediment transported into, retained in, and discharged from K-79 Reservoir. CHARACTERISTICS OF THE DRAINAGE AREA The drainage area of K-79 Reservoir is the headwaters of Kiowa Creek. That part of the Kiowa Creek basin upstream from K-79 Reservoir has an area of 3.2 square miles. (See fig. 1.) GEOLOGY AND DRAINAGE Only two geologic formations have a direct influence on present erosion and sedimentation in the K-79 drainage area. The Castle Rock Conglomerate, of Oligocene age, is a conglomeratic arkose that occurs principally as caprock in the forested divide areas in the southwestern part of the area. The Dawson Arkose, of Late Cretaceous and Paleocene age, is predominantly arkosic sandstone and clay and is the surface material in most of the basin. Upstream from K-79 Reservoir, Kiowa Creek has an average fall of about 100 feet per mile, and altitudes range from about 7,600 feet in the forested headwaters to about 7,290 feet at the reservoir site. Channels in most of the southwestern half of the K-79 drainage area are broad grassy drains. From the extreme headwaters to State Route 157, the channel generally has a grass cover and shows little evidence of rapid erosion. At the highway, the channel type changes abruptly to a well-defined, actively eroding channel. Bank erosion and bank caving are common between Route 157 and a small reservoir about three-eighths of a mile downstream. Figure 2 shows the channel of Kiowa Creek immediately downstream from Route 157. Conspicuous erosion immediately downstream from reservoirs commonly is attributed to the "de-silting" of the water and the release of relatively clear water. The channel downstream from K-79 Reservoir has steep banks and has a deep scour area immediately downstream from the outlet tube. The general characteristics of the channel downstream from the reservoir, however, do not appear to be related K-79 RESERVOIR, KIOWA CREEK BASIN, COLO.

Fluvial sediment in the drainage area of K-79 reservoir, Kiowa Creek basin, Colorado