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' HYDROLOGY <br /> South Boulder Creek has a tributary drainage area of approximately 132 square miles at <br /> its confluence with Boulder Creek. From its headwaters at the Continental Divide, <br /> South Boulder Creek flows in an easterly direction through relatively steep, narrow <br /> mountain canyons for nearly 22 miles whereupon it emerges onto the high plains at <br /> ' Eldorado Springs, Colorado. From Eldorado Springs, the creek flows northeasterly <br /> through a moderately wide valley over a distance of about 9.7 miles to its confluence <br /> ' with Boulder Creek. <br /> ' Gross Reservoir is a water storage facility located on South Boulder Creek approximately <br /> 3.5 miles upstream of Eldorado Springs. The reservoir was constructed in 1955 and has <br /> a storage capacity of 43,000 acre-feet. The reservoir is operated by the Denver Water <br /> ' Department and provides conservation storage for municipal use. It is not known <br /> whether Gross Reservoir has any flood storage capacity available. A U.S. Geological <br /> ' Survey stream gauging station is situated approximately 6.7 miles downstream of the <br /> reservoir. The drainage area at the gauging station is approximately 109 square miles. <br /> The impact of Gross Reservoir was evaluated by G&O by routing storm runoff through <br /> the reservoir pool. The G&O study modeled South Boulder Creek using the EPA <br /> Stormwater Management Model (SWMM). Flood peaks along South Boulder Creek <br /> ' were defined in SWMM using a 6-hour duration storm event. The 100-year flood of <br /> South Boulder Creek was routed in SWMM through Gross Reservoir, located upstream <br /> of the study area, which significantly reduced downstream flood peaks (G&O, 1986). <br /> The 100-year flood peak at U.S. Highway 36 is reported to be 6,160 cfs. <br /> 1 <br /> The peak flood of record at the gauging station downstream of Gross Reservoir occurred <br /> in 1938 and was estimated to be 7,390 cubic feet per second (cfs). However, it is also <br /> important to note that the gauge was not operational in 1894 when major flooding <br /> ' occurred in the Boulder Area and in the South Platte River basin. <br /> -la <br />