Coarse-sediment distribution as evidence of an elevation limit for flash flooding, Bear Creek, Colorado

Bear Creek is a tributary of the South Platte River in central Colorado. The stream flows east from an elevation of 4348 m at the Continental Divide to the mountain front at 1670 m. It thus encompasses the 2300 m elevation limit for substantial rainfall flooding in the Colorado Front Range proposed by Jarrett. Maximum paleoflood discharges estimated from flood deposits at four sites along Bear Creek demonstrate a consistent decrease in unit discharge with increasing elevation and support the hypothesis of an upper elevation limit for rainfall floods. The unit discharge values were used to explain coarse-sediment distribution along Bear Creek. Measurements of coarse-grained channel sediment at 19 sites along the creek indicate a decrease in particle size in flood deposits with increasing elevation, as well as a decrease in the size of clasts introduced to the main channel along tributaries. These changes in grain size are hypothesized to reflect changes in the competence of channel transport as a result of snowmelt-dominated versus rainfall-dominated discharge regimes above and below 2100 m elevation. Calculations of flow competence versus entrainment thresholds for the deposits may support this interpretation. One of the geomorphic implications of the elevation limit on flash flooding is a reversal of the usual downstream-fining trend in coarse channel sediments.