The Push and Pull of Alluvial Ridges: Implications for Avulsing Rivers and Alluvial Stratigraphy

River avulsions, the wholesale relocations of flow into new pathways on floodplains, are a major control on fluvial system morphodynamics and alluvial stratigraphy. Avulsions occur when a channel is superelevated, or perched, above its neighboring floodplain. During avulsions, extant alluvial ridges should repulse advancing channels, but rivers are often attracted to their parent or relict channels. The apparent contradiction of rivers as both repulsors and attractors to flow suggests a bimodal dynamic, appearing at different times and locations. Here, we explore how this bimodal dynamic affects fluvial behavior and resulting alluvial morphodynamics. We created a reduced-complexity 2D cellular model of fluvial evolution on a megafan in a foreland basin. The model operates over large temporal and spatial scales (10¹-10⁷ years; 10⁰-10² km). Channel-bed elevation is adjusted according to fluvial diffusion, allowing exploration of the transient dynamics of sediment transport and knickpoint propagation. Avulsions occur when sufficient superelevation thresholds are reached, and pathfinding occurs as weighted random walk or follows steepest-descent. Floodplain deposition scales with distance down-basin reflecting increasing concentrations of suspended fines, and laterally reflecting depth-dependent controls. Our model results generate distinct regions dominated by either progradational or reoccupational avulsions, with important implications for topographic development. Differential overbank deposition contributes to floodplain annealing and provides some control on avulsion behavior. We also demonstrate how the bimodal dynamics of channel repulsion and attraction can have important impacts on fluvial morphodynamics. These results compare favorably to planform remote sensing data available via Google Earth Engine in the modern Andean and Himalayan foreland basins.