Seasonal Changes in Bed Elevation in a Step-Pool Channel, Rocky Mountains, Colorado, U.S.A.

Abstract

Scour and fill patterns at East St. Louis Creek, Colorado, were investigated via repeat, detailed surveys of the channel bed at 11 cross sections during the 1995 snowmelt season. Spatial variability was remarkably high, with significant differences in cross section scour and fill patterns over distances as short as 0.5 m. Most sites had small net changes in bed elevation, both daily and over the entire runoff season. The data and observations indicate the presence of small pulses of fine material that are temporarily deposited on top of the channel pavement in wider areas of the channel and near woody debris complexes. Scour and fill are primarily limited to the finer material of such pulses. ANOVA analysis indicates that although discharge was important in predicting changes in bed elevation, the relationship between discharge and bed mobility is complicated by the effects of local channel morphology and a slight hysteresis. Regression analysis shows that variations in channel width determine where finer sediments are deposited, and therefore the locations of greater change in bed elevation. The proximity of morainal ridges and boulders to the channel edge locally influence the channel width and also the distribution of woody debris complexes. Results of this study suggest that the channel morphology and sediment transport along some reaches of small, high-gradient streams in watersheds with a glacial history may not respond as substantially to changes in discharge characteristic as do other types of alluvial channels.