Alluvial river-channel width: transient adjustment and dynamic equilibrium

Abstract. Alluvial river channels naturally widen and narrow as large floods scour banks and smaller ones supply sediments that help build bars and channel margins. Despite substantial advances into the controls on the equilibrium width of river channels, relatively little theory underpins our knowledge on transient river-channel-width evolution. Such a knowledge gap inhibits us from predicting the impacts of present-day nonstationary hydrology on river-channel stability and geomorphic change. Here we present a unified approach to model transient channel widening, via erosion of cohesive banks and mobilization of noncohesive clasts, and narrowing, via lateral diffusion of sediment that attaches to the banks. The resultant model can take a full hydrograph as input, allowing the hydraulic geometry and associated "geomorphically effective" water discharge to emerge dynamically. Stable widths develop via the inverse relationship between channel width and flow depth, and therefore, shear stress on the channel margins. Equilibrium solutions closely approximate data and theory on channels with both gravel and mud banks, and we compare transient solutions to the rapidly widening Minnesota and Cannon Rivers (Minnesota, USA) and the narrowing Green River and Diamond Fork (Utah, USA). Documented Python source code to run these computations is available from GitHub and Zenodo via the "OTTAR" package, and may be installed via pip from PyPI.