PROCESS AND FORM IN ALLUVIAL RIVERS

J. B. Southard's outstanding contributions to date include systematic studies of channel flows over mobile beds, particularly with respect to velocity structure, geometry of flow-related bedforms, and patterns of downstream fining in gravel systems. A compilation of observations of natural alluvial channels representing a wide range of sediment caliber and discharge conditions are examined in the context of these studies to reveal links between process and form in alluvial rivers. Gravel rivers at bankfull discharge are typically configured to be near the critical value of the Shields number, a dimensionless measure of bed shear stress, required for significant sediment transport. This state can be invoked in the interpretation of downstream fining of bed material in an individual, gravel river. Sand and silt rivers, in contrast, exhibit characteristic Shields numbers much higher than the critical value, which indicates that the stable geometries of such channels are constrained by the transport capacity of formative flows, not their competence. Based on a comparison with laboratory observations, alluvial channel beds are interpreted to be mobile during bankfull discharge. Coupling this perspective with boundary conditions for sediment-transporting flows suggests that nominal friction coefficients for formative discharges can be predicted from a simple function of, among other factors, channel slope. The ideas explored here about process and form in complex alluvial channels are necessarily simple, and show how the results of careful laboratory studies can be applied on geomorphological scales to natural settings representing a wide range of environmental conditions.