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" J Since 1905, 2,683 stallon-)'ears of record have been collected at 120 gages In the Colorado RIver Basin above 6,200 ft. Unll discharge is less than 100 ft3/slml2 for small baSins above an e1evatfon of about 7,000 It. Unit discharge Is less than 20 ft3/s1 mil for basins larger than about 200 mlZ. Below about 7.000 It, maxlmum unit dlsdiarge In the Colorado River Basin In Colorado ill about 1.600 IPIsImi2 from Intense rainstorms In small basins less than about 20 to 30 ml2. Studies have demonstrated that dams on alluvial rivers can have major effects on downstream hydrologic, sediment, water.quality, channel, riverine-habitat conditions. A simple analysiS Indicates that suspended-sediment concentratlons and sediment 1Oac1ll1n Muddy Creek are s1mDar for before and after closure of Rltschard Dam. Continued monitoring and analyZing data In Muddy CI'llek are needed to estimate and mitigate potential changes from Rllschard Dam. , . An assessment was made of the likelihood that the PMF.based hydrology (100-year and PMF can occur In the study area. The probability that the PMF.based 100-year flood of 1 0.000 ft3/s would not have occurred In the past 10.000 years In Muddy Creek BasIn Is 2.2x1lT". The probabhlty that the PMF would not have occurrild in Muddy Creek Basin In 10,000 years Is 6.7x1().03 and the probability that the PMF would not have occurred In the Colorado River Basin upstream from the Blue RIver In 10,000 years Is 1.4x10-21. These small probabilities help demonstrate that paleoflood evidence for the PMF.based ll00ds should have been Identified If nolln the Muddy Creek Basin, then the Colorado River Basin upstream from the Blue River. Thus, It raises further question as to the magnitude of the PMF estimate for Muddy Creek. Flood-frequency relations, which Incorporated the paleoflood data, were developed for the three gages on Muddy Creek and at Ritschard Dam. The 100-yr lIood estimate ranges from about 1 ,100 ft3/s at the most upstream gage (near Kremmling), 1,400 ft3/s at the above Antelope Creek gage, 2,1 00 ~/s at Ritschard Dam, anCl 2,200 ft3/s for the at Kremmling gage, Extrapolation 01 flood-frequency relations for extreme floods are provided to help place extreme ffoOds In perspective with today's understanding of flood hydrology (National Research Council, 1985; Jarrett and Costa, 1988). The recurrence Interval for the maximum paleoflood at each gaged site on Muddy Creek ranges from about 3,000 to 4,000 }Iears. The 10,OOO-yr flood estimate for Muddy Creek Is about 3,500 ft3Is for the near Kremmling gage, 5,000 fWs for the above Antelope Creek, 10.000 IPIs at Ritschard Dam, and 8,000 ft3/s for the at KremmUng. At Rilschard Dam, the PMF estimates are 24,000 to 84,000 ft3{s (Boyle engineering Corporation. 1990, 1994). Using the flood-frequency relations developed In this study. the 1994 PMF estimate of 24,000 ft3/s has a recurrence Interval of about 10 mDlion rears. The maximum paleoflood of 5,000 fl3/s In at least 10,000 years at RllSchard Dam Is 17 percen of the 1994 PMF esllmate. The large differences between the PMP/PMF-based hydrology and Interdisciplinary paleoflood ' ' hydrology for Muddy Creek and numerous other rivers In Colorado and the west8m United States Indicate the need to continue to Improve the understanding of IIood hydrometeorology. The most likely causes of excessive PMPIPMF estimates In the Colorado Rocky Mounlalns are: (1) transposition of disfant extreme rainstorms; (2) preclpltatlon-maximlzallon techniques; (3) remoteness from moisture sources: and (4) overestimallon of the areal extent of extreme reinstonns. New insight ollhe hydrometeorology of flooding gained from interdisciplinary paleoflood Invesllgations Improves flOod-frequency relations and provides information thaI can be used by dam.safety engineers and managers to help make a rlsk.based approach for hydrologic aspects of dam safely. I EO 'd ~EOS9EZ 'ON XII.:I 0.:10 ES 018 HH8 SE:Sl IH.:I 96-E -A~ - -, .-~. --