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  1. ABSTRACT The rarely witnessed process of river avulsion repositions channels across floodplains, which influences floodplain geomorphology and stratigraphic architecture. The way avulsions redirect water and sediment is typically generalized into one of two styles. Avulsions proceeding through rapid channel switching and producing little to no floodplain disturbance are annexational, while those that involve sequential phases of crevassing, flooding, and eventual development of a new channel are progradational. We test the validity of these avulsion style categories by mapping and characterizing 14 avulsion events in Andean, Himalayan, and New Guinean foreland basins. We use Landsat data to identify how avulsions proceed and interpret the possible products of these processes in terms of geomorphic features and stratigraphy. We show that during annexation the avulsion channel widens, changes its meander wavelength and amplitude, or increases channel thread count. During progradation, avulsion channels are constructed from evolving distributary networks. Often beginning as crevasse splays, these networks migrate down the floodplain gradient and frequently create and fill ponds during the process. We also see evidence for a recently defined third avulsion style. Retrogradation involves overbank flow, like progradation, but is marked by an upstream-migrating abandonment and infilling of the parent channel. Avulsion belts in thismore »study range from 5 to 60 km in length, and from 1 to 50 km in width. On average, these events demonstrate annexational style over 22.4% of their length. Eleven of 13 events either begin or end with annexation, and seven both begin and end with annexation. Only one event exhibited progradation over the entire avulsion-belt length. While there are many documented examples of purely annexational avulsions, we see little evidence for completely progradational or retrogradational avulsions, and instead suggest that a given avulsion is better envisioned as a series of spatiotemporal phases of annexation, progradation, and retrogradation. Such hybrid avulsions likely produce significantly greater stratigraphic variability than that predicted by the traditional end-member model. We suggest that a time-averaged, formation-scale consideration of avulsion products will yield more accurate characterizations of avulsion dynamics in ancient fluvial systems.« less
    Free, publicly-accessible full text available June 1, 2023
  2. Global satellite observations reveal topographic and climatic controls on river avulsions.
    Free, publicly-accessible full text available May 27, 2023
  3. 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
    Free, publicly-accessible full text available January 1, 2023
  4. 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.
    Free, publicly-accessible full text available January 1, 2023