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  1. Abstract. River avulsions are an important mechanism by which sediment is routed andemplaced in foreland basins. However, because avulsions occur infrequently,we lack observational data that might inform where, when, and why avulsionsoccur and these issues are instead often investigated by rule-basednumerical models. These models have historically simplified or neglected the effects of abandoned channels on avulsion dynamics, even though fluvialmegafans in foreland basins are characteristically covered in abandonedchannels. Here, we investigate the pervasiveness of abandoned channels onmodern fluvial megafan surfaces. Then, we present a physically basedcellular model that parameterizes interactions between a single avulsingriver and abandoned channels in a foreland basin setting. We investigate how abandoned channels affect avulsion setup, pathfinding, and landscapeevolution. We demonstrate and discuss how the processes of abandoned channel inheritance and transient knickpoint propagation post-avulsion serve to shortcut the time necessary to set up successive avulsions. Then, we address the idea that abandoned channels can both repel and attract future pathfinding flows under different conditions. By measuring the distance between the mountain front and each avulsion over long (106 to 107 years) timescales, we show that increasing abandoned channel repulsion serves to push avulsions farther from the mountain front, while increasing attraction pulls avulsions proximally. Abandoned channels domore »not persist forever, and we test possible channel healing scenarios (deposition-only, erosion-only, and far-field-directed) and show that only the final scenario achieves dynamic equilibrium without completely filling accommodation space. We also observe megafan growth occurring via ∼100 000-year cycles of lobe switching but only in our runs that employ deposition-only or erosion-only healing modes. Finally, we highlight opportunities for future field work and remote sensing efforts to inform our understanding of the role that floodplain topography, including abandoned channels, plays on avulsion dynamics.« less
  2. Abstract The process of river avulsion builds floodplains and fills alluvial basins. We report on a new style of river avulsion identified in the Landsat satellite record. We found 69 examples of retrogradational avulsions on rivers of densely forested fluvial fans in the Andean and New Guinean alluvial basins. Retrogradational avulsions are initiated by a channel blockage, e.g., a logjam, that fills the channel with sediment and forces water overbank (dechannelization), which creates a chevron-shaped flooding pattern. Dechannelization waves travel upstream at a median rate of 387 m/yr and last on average for 13 yr; many rivers show multiple dechannelizing events on the same reach. Dechannelization ends and the avulsion is complete when the river finds a new flow path. We simulate upstream-migrating dechannelization with a one-dimensional morphodynamic model for open channel flow. Observations are consistent with model results and show that channel blockages can cause dechannelization on steep (10−2 to 10−3), low-discharge (~101 m3 s−1) rivers. This illustrates a new style of floodplain sedimentation that is unaccounted for in ecologic and stratigraphic models.