Bed load texture in an alpine gravel bed river

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.