Patch dynamics and stability in steep, rough streams

The beds of steep streams are typically composed of relatively immobile boulders and more mobile patches of gravel and cobbles. Little is known about how variability in flow and sediment flux affect the area, thickness, composition, and grain mobility of sediment patches. To better understand patch dynamics, we measured flow, sediment transport, and bed properties in two steep channels. Patches close to the thalweg varied in area, thickness, and grain size, whereas those outside the thalweg did not. Local variations in transport of several orders of magnitude occurred, even on a patch with a spatially homogeneous grain size distribution. During moderate flow events, partial to selective transport dominated on the entire channel bed and all individual patches. Tracer particles moved freely between different patch classes (e.g., fine and coarse patches exchanged particles), and relatively fine sediment on all patch classes began motion at the same shear stress. Therefore, the selective transport observed for the entire bed was not a result of the preferential transport of only fine patches, but the high relative mobility of finer sediment on all patches. Our results suggest that local flow and sediment supply, and not spatial grain size variations, were the primary drivers of local bed load transport variability. The use of reach-averaged flow properties to understand local patch dynamics may not be applicable.