Abstract
River avulsions are an important mechanism by which sediment is routed and emplaced in foreland basins. However, because avulsions occur infrequently, we lack observational data that might inform where, when, and why avulsions occur and these questions are instead often investigated by rule-based numerical models. These models have historically simplified or neglected the effects of abandoned channels on avulsion dynamics, even though fluvial megafans in foreland basins are characteristically covered in abandoned channels. Here, we investigate the pervasiveness of abandoned channels on modern fluvial megafan surfaces. Then, we present a physically based cellular model that parameterizes interactions between a single avulsing river and abandoned channels in a foreland basin setting. We investigate how abandoned channels affect avulsion set-up, pathfinding, and landscape evolution. 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 do 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 ~O:105 year 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.
Highlights
Avulsions, the wholesale relocations of rivers into new positions on their floodplains, are a primary control on how water and sediment are routed through alluvial landscapes (Mackey and Bridge, 1995)
We demonstrate that abandoned channels, their interactions with future pathfinding flows, and the way they are removed from the floodplain are all important controls on avulsion locations, dynamics, and resulting foreland basin deposition and geomorphology that should be considered in future models and studies
Abandoned channels are pervasive on megafans in modern foreland basin settings
Summary
The wholesale relocations of rivers into new positions on their floodplains, are a primary control on how water and sediment are routed through alluvial landscapes (Mackey and Bridge, 1995). We present observational data on the channelization of fluvial megafan surfaces in alluvial foreland basin settings and we use these observations to motivate a physically based numerical model that parameterizes interactions between an avulsing river and abandoned channels in a subsiding basin. Abandoned channels were usually visible in satellite imagery, but in areas with dense tree canopies, we looked for channel-like pathways delineated by differences in coloration against the adjacent floodplain These differences result from the historical presence of 75 an active channel (Bernal et al, 2011); abandoned channels may have coloration that is lighter (due to sediment emplacement; Valenza et al, 2020) or darker (due to increased vegetative density associated with additional standing or groundwater). We used the topographic data in tandem with the visual data to confirm that a cell contained an abandoned channel
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