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<br />I. <br /> <br />. <br /> <br />. <br /> <br />Friday, July 12 <br /> <br />1:15 p.m.-2:45 p.m. <br /> <br />Paleohydrologic and Geomorphic Evidence of <br />an Elevation Limit for Flash-Flooding <br />Bear Creek, Colorado <br /> <br />Michael M. Grimm <br />City of Ft. Collins <br />Stormwater Utility <br /> <br />Robert D. Jarrett <br />U.S. Geological Survey <br />Water Resources Division <br /> <br />and <br /> <br />Ellen E. W ohl <br />Colorado State University <br />Department of Earth Resources <br /> <br />Bear Creek is a tributary of the South Platte River in central Colorado. The stream flows east <br />from an elevation of 4,348 m (14,264 ft) at the continental divide to the mountain front at <br />1615m (5477 ft). It thus encompasses the 2300 m (7500 ft) elevation limit for substantial <br />rainfall flooding in the Colorado Front Range proposed by Jarrett. The distinct spatial <br />distribution of rainfall-induced flash floods along channels in the Rocky Mountains facilitates a <br />study of the geomorphic role of these floods by providing contiguous channel reaches affected <br />and unaffected by rainfall floods. Measurements of coarse-grained channel sediment at 19 sites <br />along Bear Creek indicate a decrease in the deposits with increasing elevation as well as a <br />decrease in the size of clasts introduced to the main channel along tributaries. These changes <br />in grain size are hypothesized to reflect changes in channel transport competence as a result of <br />snowmelt-dominated versus rainfall-dominated discharge regimes above and below the rainfall <br />flood elevation limit. One of the geomorphic implications of the elevation limit on rainfall- <br />dominated discharge regimes above and below the rainfall flood elevation limit. One of the <br />geomorphic implications of the elevation limit on flash flooding is a reversal of the usual <br />downstream-fining trend in coarse channel sediments. The grain-size characteristics and spatial <br />distribution of coarse sediments above and below the flash flood elevation limit are distinctly <br />different. Maximum paleoflood discharges estimated from flood deposits in unit discharge with <br />increasing elevation and support the hypothesis on an upper elevation limit for rainfall floods. <br />These results emphasize the critical need for additional research to increase our knowledge of <br />flood hydrology, and have important implications in floodplain delineation studies, design of <br />hydraulic structures (bridges, dams, culverts), floodplain management, and related environmental <br />studies along the Front Range of the Rocky Mountains. <br /> <br />22 <br />