The size and shape characteristics of the surface bed sediments in a sequence of four pools and four riffles were investigated in a small cobble-bed stream. The riffle sediments are both coarser and better sorted than the pool sediments. These differences are explained in terms of reversal of the bottom velocity/shear stress hierarchy between pools and riffles at high flow. Riffle sediments are also less spherical than pool sediments, but the differences in shape are entirely size-dependent. Thus it appears that sorting occurs by size, but not by shape, as sediments are deposited in pools and riffles.

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