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I I I I I I I I I I I I I I I I I I I Study Area - flOOD HISTORY - The proposed 8.4 mile study reach for this project extends from the Colorado and Southern Railroad (C&S R.R.) crossing, located approximately 1.2 miles above the confluence with Boulder Creek, to the community of Eldorado Springs. Floods in the South Boulder Creek basin are produced by periods of intense rainfall associated with either local or general storm systems. Typically, the thunderstorm systems which produce intense periods of rainfall occur during the spring and early summer months, frequently augmenting runoff from snowmelt. Existing development along the drainageway varies substantially throughout the study reach. In the lower reaches, between the C&S Railroad crossing and Arapahoe Road, the land west of the stream channel has been developed for commercial and industrial uses. Land east of the channel to the Boulder Feeder Canal is undeveloped. Upstream of Arapahoe Road, the areas adjacent to the drainageway contain numerous residential structures along with an existing golf course. The U.S.G.S. has maintained a stream gaging station on South Boulder Creek near Eldorado Springs since 1888, however, there have been lapses in its operation. The gage was not in operati on from October 1892 to May 1895, and from September 1901 to August 1904. It should be noted that a major flood event occurred on May 29, 1894 during the period in which records were not kept. Table 2 presents occurrences of peak discharges in excess of 1000 cfs that have been recorded at this gaging station (Reference 2). From Baseline Road upstream to State Highway 93, land use is predominantly rural-agricultural. Numerous irrigation diversion structures are located along this portion of the creek. TABLE 2 Historic Peak Discharges On South Boulder Creek at Gagin~ Station Near Eldorado Springs Drainage Area: 109 Square Miles Location: 6.7 miles downstream from Gross Reservoir and 1 mile west of Eldorado Springs Peak Discharges2 Date of Occurrence c.f.s. Above the Denver-Boulder Turnpike (U.S. Highway 36) the topography abutting the southeast side of the creek is steep, rising fairly rapidly, whereas the northwest overbank areas are quite flat. Gravel mining activity is occurring nDrthwest of the creek above the Denver-Boulder Turnpike. A cluster of residential dwellings exist near the creek between State Highway 170 (Marshall Road) and State Highway 93 (South Broadway). A mobile home park is located along the south bank of South Boulder Creek just upstream of State Highway 93. Above State Highway 93 the basin and floodplain become narrower and valley sideslopes rise more abruptly. The land use is still primarily rural-agricultural and is characterized by open pastureland in the valley bottom and low growing brush on the valley slopes. 3 Jun 1895 9 May 1900 20 Jun 1909 24 May 1914 6 Jun 1921 2 Sep 1938 21 Jun 1947 6 Jun 1949 18 Jun 1951 4 Jun 1952 7 May 1969 1,130 1,100 1,340 1,240 1,440 7,390 1,290 1,430 2,370 1,080 1,690 The community of EldoradD Springs is located at the upper end of the study reach. This community lies in bottom of the narrow, steep-walled mountain canyon. Numerous residential structures have been built along the edge of the creek. 1 Records taken frDm United States Department of Interior Geological Survey, "Water-Supply Papers 1310 and 1680" and the yearly "Water Surface Records for Colorado". 2 Only peak discharges which exceeded 1,000 cubic feet per second (cfs) are listed. 4