Beyond the endmember paradigm: quantifying changes in river avulsion style through time with remote sensing data

Avulsion studies of modern rivers and stratigraphic architecture typically divide avulsion events into two general styles based on the way the channel shifts to a new location on the floodplain. Avulsions that divert flow directly into relict channels with little to no floodplain sedimentation exhibit annexational style, whereas avulsions that create advancing distributary networks exhibit progradational style. Using annexational and progradational endmembers has allowed us to tie different processes to specific morphologies and sediment packages, but these models also limit our perception of hybrid styles. Using Landsat data from Andean, Himalayan, and New Guinean foreland basins to observe and map avulsions, we quantify the land-cover changes visible through a Tasseled Cap Transformation that occur to the channel(s) and floodplain through time. Here we present results showing that the style exhibited by a given avulsion often varies through time. Events that begin as prograding distributary networks often shift to annexation if they encounter an accommodating relict channel. In other cases, we observe annexational events phasing into progradation if the annexed channel becomes overwhelmed by new discharge. This results in overbank flow and new channel construction through a progradational distributary network. When avulsions exhibit large fluctuations in style through time progradation and annexation occur in distinct spatial and temporal domains. We find that of 66 mapped avulsions, 21 exhibit some combination of both annexation and progradation. These results have important implications for the interpretation of fluvial stratigraphy.