Short Communication: Field Data Reveal That the Transport Probability of Clasts in Peruvian and Swiss Streams Mainly Depends on the Sorting of the Grains

Abstract. We present field observations from coarse-grained streams in the Swiss Alps and the Peruvian Andes to explore the controls on the probability of material entrainment. We calculate shear stress that is expected for a mean annual water discharge and compare these estimates with grain-specific critical shear stresses that we use as thresholds. We find that the probability of material transport largely depends on the sorting of the bed material, expressed by the D96∕D50 ratio, and the reach gradient but not on mean annual discharge. The results of regression analyses additionally suggest that among these variables, the sorting exerts the largest control on the transport probability of grains. Furthermore, because the sorting is significantly correlated neither to reach gradient nor to water discharge, we propose that the granulometric composition of the material represents an independent, yet important control on the motion of clasts in coarse-grained streams.