Differential bed sedimentology and the maintenance of riffle-pool sequences

Riffle-pools are the characteristic reach-scale bedforms of gravel- and mixed-bedded channels of low to moderate slope. The purpose of this paper is to clarify several aspects of the sedimentology of riffle-pool sequences, and to suggest how these both reflect and control riffle-pool stability in the presence of spatial differences in the turbulent near-bed flow field. High-frequency velocity measurements obtained using an array of electromagnetic current meters, are combined with results from grid and transect sampling of bed sediments from 9 sites on 3 small upland rivers in N. Exmoor, U.K. Riffle sediments are shown to possess greater development of microtopographic structuring than adjacent pools, which enhances riffle stability, and which may reflect greater turbulent kinetic energy there over most of the flow range. Data on surface sediment arrangement and turbulent flow characteristics can be combined with different transport threshold equations to explain the maintenance of riffle-pool sequences with respect to a particle queuing (‘kinematic wave’) model, without recourse to a competence reversal, as conventionally assumed.