Short communication: Field data imply that the sorting (<i>D</i><sub>96</sub> / <i>D</i><sub>50</sub> ratios) of gravel bars in coarse-grained streams influences the probability of sediment transport

Abstract. Conceptual models suggest that the mobility of fluvial gravel bars is mainly controlled by sediment discharge. Here we present field observations from streams in the Swiss Alps and the Peruvian Andes to document that for a given water runoff, the probability of bedload transport also depends on the sorting of the bed material. We calculate shear stresses that are expected for a mean annual water discharge, and compare these estimates with grain-specific thresholds. We find a positive correlation between the predicted probability of transport and the sorting of the bed material, expressed by the D96 / D50 ratio. These results suggest that besides sediment discharge, the bedload sorting exerts a measurable control on the gravel bar mobility.