Upstream-migrating channel blockage causes rivers on fluvial fans to dechannelize and avulse

Abstract:

The process of river avulsion builds floodplains and fills alluvial basins on Earth and other planets. Here we report on a new style of river avulsion discovered in the Landsat remote sensing record. We found 70 examples of upstream-migrating dechannelizing avulsions on the rivers of densely forested fluvial fans in the Andean, Himalayan, and Papuan basins. These rivers dechannelize because an initial channel blockage, possibly caused by logjams or sediment accumulation, creates a bow wave and overbank flooding with a characteristic upstream-pointing chevron shape. Decelerating flow upstream of an initial blockage promotes additional sediment accumulation and wood jams, and the chevron-shaped flood zone continues to migrate upstream until the overbank flow finds a new pathway and the river avulses. Observations show that the dechannelization fronts, on average, travel upstream at a velocity of 550 m/yr and last 15 years, though some travel as fast as 2000 m/yr and last 30 years. Over 34 years of record, we observed the dechannelization of a total of 424 km of river, with some rivers showing multiple dechannelization events from similar initiation points. We simulate this dechannelization wave with a 1D morphodynamic model that solves the unsteady, nonuniform St. Venant equations for open channel flow. We impose an initial channel blockage height and use a side-weir formulation for the resulting overbank flow. Model experiments show that dechannelization ultimately depends on the size of the initial blockage height relative to the channel depth and the bankfull Shields parameter. Consistent with remote sensing observations, the rivers with relatively steep slopes and small drainage areas are more likely to dechannelize for a given blockage height because the bow wave area is large compared to the unit discharge, generating significant overbank flow. These results illustrate a new way for rivers to avulse and a style of floodplain construction in densely forested regions that is unaccounted for in models. Given this process the rise of large trees in the mid-Devonian would have promoted this style of avulsion and diversified floodplain deposition in the stratigraphic record.