Variable source areas of bedload in a gravel-bed stream

Bedload texture is dependent on the spatially variable availability of mobile surface bed material, distributed in patches or in other bedforms. We measured bedload transport continuously with an automatic bedload station on the gravel-bedded Tordera River. We also studied bed patchiness from prc-flood vertical aerial photographs in a 40 m section upstream of the bedload station. The results suggest that patches are the main source of sediment during the beginning of the flow event. The median bedload size, in low-magnitude events, depends on the percentage of total area occupied by patches of a given size. Patch areal extent and texture depend on the magnitude and general character of the previous flow event.