Geomorphic implications of hydroclimatic differences among step-pool channels

Abstract

We used nine step-pool channel segments from six regions to examine the magnitude, frequency, and duration of flows hypothetically capable of mobilizing the D50 and D84 of streambed sediment. The six regions examined represent different hydroclimatic regimes: Switzerland (snowmelt runoff and moderate-intensity summer rainfall), Puerto Rico (runoff from high-intensity, short-duration rainfall throughout the year), Colorado (snowmelt runoff), Oregon and northern California (low-intensity winter rainfall runoff), and Idaho (snowmelt and rain-on-snow). We hypothesize that differences in hydroclimatology will correlate with differences in the magnitude and frequency of sediment mobilization such that the tropical site with high-intensity rainfall and abundant total precipitation will have the greatest magnitude and frequency of sediment mobilization, whereas the snowmelt site will have the lowest magnitude and frequency of sediment mobilization but relatively high values of cumulative sediment mobility as a result of longer duration of flows above the threshold of sediment mobilization. The results partially support this hypothesis. The tropical site does have significantly greater magnitude and frequency of exceedence for flows hypothetically capable of mobilizing coarse sediment and the snowmelt site has substantially lower values, but the tropical site also has the highest value of cumulative sediment mobility. ANOVA models indicate that total precipitation and precipitation intensity are the best predictors of magnitude and frequency of exceedence of threshold discharge and cumulative mobility of coarse sediment.