Does the Threshold of Sediment Motion Constrain the Width of an Incising Laboratory River?. Issue 22 (15th November 2022) - Ashley, T. C.
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. Plain Language Summary: River channel width influences the rates and locations of large‐scale geomorphic change. Recent studies argue that channel width is set by a balance between fluid forces and gravity acting on sediment particles that make up the banks and bed. Here, we test whether this principle constrains the width of a disequilibrium laboratory river. Width remains roughly constant throughout the experiment, diverging from physical theory but mirroring observations of natural rivers. Based on this result, we suggest that many rivers on Earth's surface could be far from equilibrium. Key Points: Sediment discharge approaches zero in a laboratory river with no sediment supply Widths depart from near‐threshold model prediction Comparison with global database indicates many rivers could be far from equilibrium