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 Colorad o. The stream flows east from an elevation of 4348 m at the Continenta l Divide to the mountain front at 1670 m. It thus encompasses the 2300 m elevation limit for substantial rainfall flooding in the Colorado F ront Range proposed by Jarrett. Maximum paleoflood discharges estimate d from flood deposits at four sites along Bear Creek demonstrate a con sistent decrease in unit discharge with increasing elevation and suppo rt the hypothesis of an upper elevation limit for rainfall floods. The unit discharge values were used to explain coarse-sediment distributi on along Bear Creek. Measurements of coarse-grained channel sediment a t 19 sites along the creek indicate a decrease in particle size in flo od deposits with increasing elevation, as well as a decrease in the si ze of clasts introduced to the main channel along tributaries. These c hanges in grain size are hypothesized to reflect changes in the compet ence of channel transport as a result of snowmelt-dominated versus rai nfall-dominated discharge regimes above and below 2100 m elevation. Ca lculations of flow competence versus entrainment thresholds for the de posits may support this interpretation. One of the geomorphic implicat ions of the elevation limit on Bash flooding is a reversal of the usua l downstream-fining trend in coarse channel sediments.