Variable source areas of bedload in a gravel-bed stream

Bedload texture is dependent on the spatially variable availability of mobi le surface bed material, distributed in patches or in other bedforms, We me asured bedload transport continuously with an automatic bedload station on the gravel-bedded Tordera River. We also studied bed patchiness from pre-fl ood vertical aerial photographs in a 40 m section upstream of the bedload s tation. The results suggest that patches are the main source of sediment du ring the beginning of the how event. The median bedload size, in low-magnit ude 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 ge neral character of the previous flow event.