Transport of coarse, heterogeneous debris in a natural stream under a wide range of flows usually results in a remarkably stable, undulatory bed profile, which manifests an in transit sorting process of the bed material. In general, finer material representative of the bulk of the normal bed load resides in the deep sections, or pools, below flood stages. At high flows, pools may scour to immovable boulders or bedrock. Coarser material transported at more infrequent flows forms the shallow sections, or riffles. Above a flow threshold, the pool fill material is often scoured and deposited in part over the riffles (Lane and Borland, 1954; Andrews, 1977). Interactions among coarse particles in motion in turbulent flow tend to concentrate them in groups with the coarsest particles at the surface (Langbein and Leopold, 1968). Incipient accumulations of coarse particles may be perpetuated by altering the flow conditions which influence bed load transport.
As flow increases, submergence of the riffle-pool bed topography modifies its effect on the local hydraulic conditions. At low flow, mean volocity and water surface slope are greater, and mean depth is less at a riffle than at a pool. Competence is greater at the riffle. As the stage increases, the water-surface profile tends to even out as the hydraulic gradient over a riffle decreases and that over a pool increases (Leopold and others, 1964). Corresponding values of velocity and depth tend to converge, although depth often less so (Richards, 1976; Lisle, 1976; Andrews, 1977). The convergence of respective values of water-surface slope over a riffle and pool and the greater depth of the pool cause mean shear stress, τ = γRSE (where γ is the specific gravity of water, R is the hydraulic radius or approximately the mean depth, and SE is the energy gradient), to increase more rapidly at the pool (Leopold and others, 1964). As a result, competence as measured by velocity or bottom shear stress should become more evenly distributed over a riffle and pool (Richards, 1976) or even reversed in. hierarchy at high flow (Gilbert, 1914; Keller, 1971).