Intermittent bank collapse as an inherent control on meandering river morphology

Meandering rivers are a ubiquitous feature worldwide, exhibiting an extraordinary variety of planform patterns. These patterns, from widely observed point bars to alternating bend widening and narrowing, provide compelling evidence of a pulsed lateral migration of meandering rivers. While these rhythmic migrations have recently been tied to intermittent bank collapses, their morphological impacts over large temporal and spatial scales remains largely unexplored. Here we investigate a globally distributed set of alluvial rivers, using satellite imagery and Fourier analysis to identify low-wavelength width variations caused by bank collapses. Our findings reveal that intermittent bank collapse enhances channel width variation, particularly in narrower and less sinuous meanders, exhibiting a positive correlation with the ratio of channel width to curvature radius. Based on observational evidence, we develop parameterized physics-based relations to optimize the balance between the reliability and numerical efficiency of modeling intermittent bank collapse. These relations are subsequently incorporated into a model of river meandering. We find that intermittent bank collapses play a crucial role in shaping the morphology of meandering rivers, accounting for the observed channel width variations, the shift in bend skewness as sinuosity increases, and the prevalence of low-sinuosity bends. The influence of bank collapses stems from their varying frequency along meander bends, thereby introducing width variations and associated curvature perturbations. Our findings elucidate a long-overlooked mechanism that drives meandering river evolution.