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1942, 1948, 1957, 1964, 1965, 1967, 1973, and 1979. Among these events, the capacity of Bootleg Reservoir (approximately 3,600 acre-feet) was completely filled in 1924, 1935, and 1973. The 1973 flood was one of the largest recorded floods on Box Elder Creek; it damaged portions of Weld County Roads 4, 8, 12, 14, 16, 22, 49, and 52 and destroyed private roads, driveways, and livestock sheds. The Urban Drainage and Flood Control District estimated that the peak discharge of the 1973 event, possibly the flood of record, may have been on the order of 6,000 cfs at Bootleg Reservoir. This estimate was based on anecdotal descriptions of reservoir of inflow and outflow rates. Estimates of rainfall for the 1973 event ranged from 5 to 6 inches in 24 hours, approximating the magnitude of the l00-year precipitation identified in the National Oceanographic and Atmospheric Administration (NOAA) Atlas for Colorado (NOAA, 1973). Another major runoff. event 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 to 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, a distance of approximately 10 miles. In a number of areas, cropland tillage has encroached 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 B. Existing imperviousness is low (generally 1 percent), which is representative of undeveloped grassland and agricultura1land. Small areas of existing development are located at the towns of Watkins and Elizabeth, Colorado. Watkins is located at 1-70 ~d Elizabeth is located in the watershed upstream of the study area. Several large-lot rural subdivisions exist in unincorporated Arapahoe County. These include Fritzler's Subdivision along the east branch of Coyote Run and Colomeadow Acres and Box Elder Creek Ranches along Box Elder Creek. 2-3 1976 by the U.S. Soil Conservation Service. The location of this facility is not readily apparent on current 1:50,000 scaie U.S. Geological Survey (uSGS) mapping of Elbert County. IS Bootleg Reservoir, a State jurisdictional dam owned by the Henrylyn Irrigation District, located in Adams County, approximately 12 miles downstream of the study area. This dam was breached in the early 1980s because of safety concerns by the State Engineer's Office (CCWCD, 1987). A detailed hydrologic analysis of the Box Elder Creek watershed upstream of Bootleg Reservoir was prepared by the U.S. Army Corps of Engineers (COE) in 1989 (COE, 1989). This analysis was part of a study examining the feasibility of rehabilitating the reservoir. To date, Bootleg Reservoir has not been rehabilitated. 1 I 1 1 1 1 1 Diversions There are no significant surface water diversions of Box Elder Creek flows in the study area, although several diversions exist upstream and downstream of the study area. Water usage within the study area consist primarily of groundwater withdrawals. 2.3.3 2.3.4 Flood History According to information provided in the Arapahoe County Flood Insurance Study (PIS), three major floods have occurred on Box Elder Creek and in nearby watersheds in recent times (FEMA, 1993). The following is an excerpt from the PIS report. "In interviews held in Watkins, Strasburg, Byers, and Deer Trail regarding flood histories on Box Elder Creek, Comanche Creek, West Bijou Creek, and East Bijou Creek, residents recalled severe damage and lost lives in floods occurring in 1905, 1935, and 1965." were given. Flood history of Running Creek (Upper Box Elder Creek) in the Elizabeth, Colorado area was documented in a Floodplain Information Report prepared for the Colorado Water Conservation Board (WRC, 1979). The information in this report attests to significant flood events along the creek in 1935, when the bridge across the creek at Elizabeth failed, and in 1965. It was reported that flooding along Running Creek was less severe than in adjoining watersheds in both the 1935 and 1965 events. In 1935, flooding was most severe to the east in Kiowa and Bijou Creeks. In 1965, flooding was far more severe to the east in Kiowa Creek and Bijou Creek, and to the west in Cherry Creek and Plum Creek. Flood history of Box Elder Creek was also documented at Bootleg Reservoir, located approximately 15 miles downstream of this study area, in a report produced by the Central Colorado Water Conservancy District (CCWCD, 1987). The report mentions that major runoff events have occurred on Box Elder Creek at Bootleg Reservoir in 1924, 1935, DENl0015184.WP5 No estimates of historic peak discharges 'I 1 ,I I 'I 1 1 1 1 I I) 1