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I J "2 Hydrologically, these two large reservoi rs have tremendous impact. The land tributary to the reservoi rs is largely occupied by Rocky Flats, resulting in large areas of undeveloped, pervlow, land. Because of the permanent nature of Rocky Flats, it was assumed that the land use of Basins A and B will not appreciably change. The operational characteristics of both reservol rs mean that the water surfaces will be normally below the crest elevation of the spi 11- ways. Because the enti re flood for the lOCi-year return frequency could be stored within one foot in Standley lake and within two feet in the Great Western Reservoir, and because it is unlikely that the reservoirs would be full, it was assumed that there would be no overflow from ei ther reservoi r due to storm runoff; howeve,-, It: was assumed that each reservoir would be making normal irrigation rele"ses durin!l the assumed storms that were studied, as discussed in Section IV, "Hyd,'ology." The remainder of the basins, with the exception of Broomfield, are presently mostly undeveloped, with some limited r.esldential and light industrial lands. Presently the land is covered with "plains" grasses with little tree and shrub type vegetation. Projected land use calls for full urbanization, wi th some areas al ready under' severe develop- mental stress (Basins C, D, E, and F). All govemmental entities, Westminster, Adams County, and Jefferson County, plan on the use of open space, parks, and recreatIonal areas that ar'e centered, In large part, on streams and creeks which course the area. For future develop- ment of the basin, and consIstent with local planning, it was assumed that forty percent of the basin below Standley L,;rke and the Great Western Reservoir would be impervious. There are 2,200 acres of open water surface in the study area, with 1,200 acres in Standley Lake, 250 acres in Great Western Reservoir, and 750 acres in the 150 to 200 lakes in the area. The open water sur-' face is now approximately four percent of the entire basin. There are also over 75 mIles of ditches and canals in the basin. It has been assumed that theIr net effect on trans-basin diversions du,-Ing most storms is not significant. Although slight, the effect of these ditches will be to lengthen the time of concentration and lower the peak runoffs. The surface geology of the area shows that Illost of th" bas in is alluvial terrace. There is some residual sandstone soil near Standley Lake, some recent alluvial deposits along Big Dry Creek, and some oelian sand and loess In some areas. The bedrock geol09Y of the area shows the Laramie formation to be generally north of Big Dr.y Creek and west of Standley Lake. The Denver formation (.~rapahoe and Da'rison "rkose) is mainly south and east of Big Dry Creek. The Big Dry Creek channel Is twenty-on.!'mll,,,s long from the northern border of Adams County to the end of the study area below Standley Lake. '';Longer when measured along existing meandering low-'flow channel.