The Push & Pull of Abandoned Channels: How Floodplain Processes and Abandoned Channel Healing Affect Avulsion Dynamics and Alluvial Landscape Evolution in Sedimentary Basins

River avulsions are an important mechanism that routes and emplaces sediment in foreland basins, and should thus control landscape evolution in these settings. Repeated avulsions over long timescales can build large (O:~103-105 km2), low gradient fluvial megafans that radially disperse sediment where rivers leave mountain-fronts. Because avulsions occur infrequently, there is a lack of observational data that might inform where, when, and why these avulsions occur and the resulting impacts on planform morphology. These questions are instead often investigated by rule-based numerical models, but these models, however, have historically simplified or neglected the effects of abandoned channels on avulsion dynamics. Since fluvial megafans are characteristically covered in remnant abandoned channel topography, it is expected that these may significantly affect resulting constructed landscapes. Here, we evaluate topographic features associated with abandoned channels on modern fluvial megafan settings and present a physically-based cellular model of a single avulsing river that interacts with abandoned channels. We demonstrate how these interactions affect large-scale landscape evolution via local avulsion set-up and pathfinding processes. We show how abandoned channel inheritance and transient knickpoint propagation post-avulsion can shortcut the time needed to set-up future avulsions. We address how abandoned channels could both repel and attract future pathfinding flows under different conditions. By measuring the distance between the mountain-front and each avulsion over long (105-107 yrs) timescales, we investigate where avulsions occur under different abandoned channel dynamic scenarios. We show that increasing abandoned channel repulsion pushes avulsions further from the mountain-front, while increasing attraction pulls avulsions proximally. We hypothesize and test possible abandoned channel healing scenarios (deposition-only, erosion-only, and far-field directed), showing that only the final scenario achieves dynamic equilibrium without filling accommodation space. Finally, we highlight opportunities for field work and remote sensing to inform our understanding of the roles that floodplain and avulsion dynamics play on large-scale landscape evolution.