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. between maximum paleofloods and PMF estimates in the Rocky Mountain region (PMF estimates from Bullard, 1986). The maximum paleoflood estimate for these studies all are the largest discharge to have occur in 10,000 years or longer. A paleoflood study was conducted by Jarrett and Costa (1988) for Olympus Dam in Estes Park, Colorado, on the Big Thompson River at Estes Park, which primarily drains Rocky Mountain National Park, They estimated that the maximum paleoflood of 5,000 ft3/s is 6 percent of the PMF of 84,000 1t3/S. For a study of Horsetooth Reservoir in the foothills near Fort Collins, Colorado, the maximum paleoflood of 2,840 ft3/s is 24 percent of the PMF (12,200 1t3/S) for Arthur s Rock Gulch (Jarrett and Waythomas, in press). The maximum paleoflood of less than 10,000 ft3/s for Bear Creek at Morrison, Colorado (Grimm, 1993) is 6 percent of the spillway capacity of 153,500 1t3/S (which is assumed to be equal to the PMF). Grimm also determined the maximum paleoflood of less than 2,000 ft3/s on Bear Creek near Evergreen Dam, which is 2 percent of the PMF of 124,000 ft3/s, The maximum paleoflood of less than 5,000 ft3/s for Muddy Creek at Ritschard Dam near Kremmling, Colorado (Jarrett, in review) is less than 20 percent of the PMF of 24,000 1t3/S. Levish and others (1994) estimated the maximum paleoflood to be less than about 90,000 ft3/S for the Santa Ynez River at Bradbury Dam in the valley between the San Rafael and Santa Ynez Mountains in Califomia, which is 22 percent of the PMF of 414,000 ft3/S. Ostenaa and Levish (1995) conducted a paleoflood study for the South Fork Ogden River at Causey Dam in the mountains west of Ogden, Utah. They estimated that the maximum paleoflood is less than 4,100 ft3/s, which is about 4 percent of the PMF of 108,600 1t3/S, House and Pearthree (1995) calculated a maximum paleoflood of about 30,000 ft3/s for Bronco Creek, which is one of the largest known . floods for a 19.3 mi2 basin in the westem United States, is located in the Hualapai Mountains of Arizona, The maximum paleoflood is 30 percent of the PMF of 98,700 ft3/s for Bronco Creek. Values of the maximum paleoflood and PMF for sites in the mountainous areas in the westem United States are shown on figure 15. The large differences between PMF and maximum floods or paleofloods, which average about 5 percent for sites in the Colorado mountains, is reason for concem. . The PMP/PMF methods and paleoflood techniques have inherent assumptions and limitations that produce uncertain flood estimates. PMP/PMF methods are based on theoretical assumptions about extreme-flood processes. In the Colorado Rockies, transposition of extreme rainstorms from distant areas that may not be similar hydrologically have been used to estimate PMP values. Paleoflood estimates are based on physical evidence preserved in channels for thousands to tens of thousands of years. Although paleoflood estimates have uncertainty, these estimates are based on physical data preserved in channels and on floodplains. Paleoflood estimates for mountain rivers have uncertainties of about 25 percent (Jarrett and Waythomas, in press). This uncertainty primarily is related to representativeness of PSis of past flood depths and possible post-flood changes in channel geometry. The HWM-PSI relations developed from 1995 and other flood data in Colorado reduce the uncertainty of paleodischarge estimates in mountain and alluvial channels, Interdisciplinary paleoflood hydrology provides long-term hydrologic data, particularly information on maximum flooding in a river basin. This information can be used in a risk- I based approach for hydrologic aspects of dam safety, Clearly, the important issues in flood hydrology are: (1) the magnitude of differences between the two methods; (2) the effect of questionable extreme rainfall and flood data on past flood-study results; (3) the need to improve the understanding of flood 30