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, Regional Paleoflood Investigation to Assess TopographIc Effects of the Divide on flooding In Eastern Colorado RECEIVED DEe f 8 1998 Palmer CoI0md0 Water Conservation Soard -J Robert D, Jarrett, U,S, Geological Survey, P,O, Box 25046, MS 412, Denver, Colorado 80225 ABSTRACT There is substantial uncertainty and controversy in estimating flood magnitude and frequency, particularly those of extreme floods, in the Rocky Mountain region. A regional paleoflood study was conducted to assist dam-safety officials and flood-plain managers in assessing the risk of large floods in eastern Colorado, particularly with respect to topographic effects on the spatial variability of rainstorms and flooding. Regional envelope curves encompassing maximum gaged and historical floods were developed for Palmer Divide streams, Paleoflood data for 134 sites on streams draining from the Palmer Divide were used to document the largest floods that have occurred during the last few hundred to several thousands of years and complement the regional envelope curve of maximum flooding, The approach was applied to Cherry and Box Elder Creeks, which drain northerly from the Palmer Divide, Maximum paleofloods from rainfall in Cherry Creek range from about 30,000 fl3ls near Franktown to about 80,000 fl3/s near Cherry Creek Reservoir. Maximum paleofloods in Box Elder Creek range from about 9,000 fl3/s near Elizabeth to about 8,700 fl3/s near Watkins (1-70), Maximum floods in Cherry and Box Elder Creek basins are substantially smaller than floods on other streams draining from the Palmer Divide, Smaller floods in these basins result from the topographic relief of the Palmer Divide and localized areas of smaller topographic relief (less than about 300 fl in about 3 miles or less) that substantially affects the spatial distribution of rainfall and maximum flooding in upslope and lee-slope areas of the Divide. Study results demonstrate that the high plains of eastern Colorado are not hydrologically homogeneous as has been long thought. The high plains needs to be divided into at least two hydrologically homogeneous regions: basins having topographic relief less than about 300 fl and more than 300 fl in less than about 3 miles distances, The elongated shape and very high infiltration in sandy basins such as Box Elder Creek basin also contribute to smaller maximum flooding, Flood-frequency relations for Cherry Creek, which incorporate paleoflood data, indicate the 10,000-year flood ranges from about 44,000 fl3/s (near Franktown) to about 75,000 fl3/s (near Melvin), Floods the magnitude of PMF values for Cherry and Box Elder Creeks plot far in excess of a 10,000-year event. The maximum paleoflood of about 75,000 fl3/s for Cherry Creek near Cherry Creek Reservoir is about 11 percent of the PMF of 662,000 fl3/s, Paleoflood data also were used to validate f1ood- frequency relations for an ungaged basin such as Box Elder Creek, Important issues related to these research results are: (1) large differences in flood hydrology for eastern Colorado using different methods; (2) the major effects of topography, including local areas of relatively small relief, and basin elongation on flood hydrology in eastern Colorado; and (3) the need to improve the understanding of flood hydrometeorology in eastern Colorado, thereby reducing the uncertainty of estimates of the magnitude and frequency of flooding, Future research in flood hydrometeorology is needed to address these issues, 1