Does the Threshold of Sediment Motion Constrain the Width of an Incising Laboratory River?

Abstract

A physically rational model for river width is critical to predict macroscopic landscape evolution driven by fluvial sediment transport. Growing evidence suggests that rivers widen until the stress exerted by the fluid on the bed surface is close to the critical entrainment stress of the bank material. In this study, we test the limits of this model as a closure assumption in dynamically evolving river systems. We consider a simple laboratory channel with a fixed water discharge, monodisperse bed material, no sediment supply, and an initial relief that was sufficiently large to guarantee a finite transport capacity. Over time, the transport capacity approaches zero through changes in channel morphology. Concurrent measurements of width and sediment load illustrate how theory developed for equilibrium rivers fails to predict transient channel processes. Similarity to first order trends in natural rivers suggest that many rivers could be far from equilibrium.