Bed load texture in an Alpine gravel bed river

Abstract

The spatial and temporal variations in bed load texture measured in a gravel bed river with a Helley‐Smith sampler are compared with its bed material texture. Most measurements were undertaken at low excess shear stress, with low bed load discharge over an armored bed. Cross‐sectionally averaged fractional bed load discharge increases with water discharge, but the explanation of the variance in fractional discharge is low (20–40%). Cross‐sectional distribution of shear velocity explains in part the spatial variation in bed load texture. Because local hydraulic parameters are weak predictors of fractional bed load discharge for the granule‐pebble range at nonequilibrium conditions, it reinforces the hypothesis that this size range, which comprises the bulk of bed load, derives from upstream sources. Temporal variations in the cross‐sectional distribution of bed load texture are shown to depend on variations in bed topography and local shear stress. We demonstrate that the dominance of granule‐pebble fractions is in part related to the hiding behavior of smaller fractions.