Bed material sorting in pools and riffles.

When rivers flow in a series of riffles and pools during periods of low flow, bed material sizes change from coarser to finer in the riffle-pool sequence. The Froude Number also changes from high to low from riffle to pool. The sorting of bed materials, with coarser material at the riffles and finer materials at the pools, is mainly due to the high bed shear stress at the riffles, where the fine materials unable to withstand the high shear stress are washed away and deposited in the pools downstream where the bed shear stress is lower. This process of sorting takes place during low discharges when the difference in velocity between that of a riffle and a pool are the greatest.-from ASCE Publications Abstracts