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Quantifying river avulsion activity from satellite remote sensing: Implications for how avulsions contribute to floodplain stratigraphy in foreland basinsABSTRACT 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 »Free, publicly-accessible full text available June 1, 2023
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.