An explanation of the occurrence of coarse sediments in riffles, and fine sediments in pools, by variations in discharge. At high discharge, high velocities would move coarse sediment rapidly through the pools, while relatively low velocities in riffles would cause its deposition there. The fine particles are winnowed from riffles and deposited in the pools. Booker et al. (2001) ESPL 26 think that velocity reversal is an inadequate explanation of pool-riffle sequences, and Thompson (2011) PPG 35, 1, 123 agrees: ‘the velocity reversal model by itself does not explain pool–riffle maintenance’.