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I I I I I I I I I I I I I I I I I I I 1973 event may have been on the order of 6,000 cfs at Bootleg Reservoir, based on anecdotal evidence of inflow and outflow rates. Another major runoff events may have occurred in 1949. Henrylyn Irrigation Company records document a large storm on June 12 and 13, 1949, that caused major damage to Lost Creek, the adjacent drainageway to the east. Aerial photography of Box Elder Creek, dated June 14, 1950, shows evidence of substantial channel erosion from what must have been a major event occurring within one or two growing seasons prior to the date of the photographs. 2.4 Vegetation Vegetation in the watershed consists primarily of native grasses typical of high plains areas, with a portion of the prairie grassland being used as pasture. About half of the study area (primarily the north half) is currently in agricultural use as cropland. Inter- mittent stands of ponderosa pine exist in the watershed upstream of the study limit. Natural riparian vegetation is evident along many of the major streams in the study area and consists primarily of grasses, with isolated areas of shrubs and trees. In general, the natural riparian vegetation is confined in the alluvial channel corridor and varies in width from 120 to 1,700 feet. A significant stand of cottonwood trees extends along Box Elder Creek from about 2 miles south of 1-70 to the southern study limit. In a number of areas, cropland tillage has encroach@d on, or totally eliminated, natural riparian vegetation along streams. 2.5 Watershed Imperviousness Imperviousness representative of existing development conditions in the study area is shown in Appendix C. Existing imperviousness is low (generally 1 percent), which is representative of undeveloped grassland and agricultural land. Small areas of existing development are located at the towns of Watkins and Elizabeth, Colorado. Watkins is located at 1-70 and Elizabeth is located in the watershed upstream of the study area. Imperviousness representative of future develollment conditions in the study area is shown in Appendix D. Future imperviousness for long-term full build-out conditions was based on input provided by the project sponsors. Future imperviousness in the portion of the watershed from I mile south of 1-70 to the north end of the study area was based on the City of Aurora's Comprehensive Plan. Comprehensive planning has not yet been completed in the remainder of the study area within Arapahoe County. It was determined by the project sponsors that the study objectives could be satisfied without having detailed planning information in Arapahoe County, as long as the watershed was modeled on the basis of a reasonable assumption of DENlOOlSI84.WPS 2-4