Origin of Step-Pool Systems in Mountain Streams

Mountain streams very often present a step-pool aspect. Antidune formation alone; dispersion and sorting theory or velocity reversal as advocated for the origin of riffle-pool sequences do not explain how steps and pools form. Experiments have been conducted to clarify their origin. It was found that the step-pool aspect appears after high flows had deformed the bed. During the presented tests a self armoring process occurred which resulted in a stabilization of the bed. When coarsening of the top layer was little classic antidunes resulted. An important coarsening was accompanied by a structuring of the bed into roughness elements whose spacing did correspond to maximum flow resistance. The substantial increase in resistance was an essential factor in bed-stabilization.