Influences of Increased Sand Delivery on the Morphology of Sand and Gravel Channels1

A flume study was conducted to examine (1)changes in the particle‐size distribution of sediments in riffles due to the proportion of sand in transport and the total rate of bedload transport at the time the riffle is deposited and (2) the effect of high sand transport rates on the stability of gravel riffles. The median particle size of sediment deposited in the riffle was larger than that of the sediment in transport. Small but significant (a = 0.05) decreases in the median particle size of riffle sediments resulted as the sand‐to‐gravel ratio. Increased concentrations of sand in transport caused previously stable gravel riffles to undergo scour. These results, in combination with information from other studies, suggest that an alluvial channel with pool‐riffle sequences and with sand and gravel beds may respond to an increased delivery of sand by reducing form roughness. Form roughness can be reduced by degrading riffles and filling pools. Subsequent responses may be increases in width‐to‐depth ratio and slope.