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 examplesof retrogradational avulsions on rivers of densely forested fluvial fans in the Andeanand New Guinean alluvial basins. Retrogradational avulsions are initiated by a channelblockage, e.g., a logjam, that fills the channel with sediment and forces water overbank(dechannelization), which creates a chevron-shaped flooding pattern. Dechannelizationwaves travel upstream at a median rate of 387 m/yr and last on average for 13 yr; manyrivers show multiple dechannelizing events on the same reach. Dechannelization endsand the avulsion is complete when the river finds a new flow path. We simulate upstreammigratingdechannelization with a one-dimensional morphodynamic model for openchannel flow. Observations are consistent with model results and show that channelblockages can cause dechannelization on steep (10−2 to 10−3), low-discharge (∼101 m3 s−1)rivers.
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