Downstream Controls on Coastal Plain River Avulsions: A Global Study

The avulsion frequency of coastal-plain rivers is primarily governed by the rate at which channels become superelevated over neighboring plains, which is itself controlled by multiple factors. Notably, the importance of wave and tidal processes, the rates of relative sea-level (RSL) change, and the bathymetry of the receiving basin are thought to affect channel morphodynamics and channel-mouth progradation, thereby controlling streambed aggradation and influencing the avulsion frequency and drainage density of coastal plains and deltas. This work tests the significance of these downstream factors on the avulsion histories of 57 Holocene lowland river systems. A quantitative analysis is performed of relationships between variables that quantify downstream controls and estimations of avulsion frequency, based on the number of avulsion events, active or abandoned channel paths, and delta lobes; measures of spatiotemporal avulsion "density" are also derived by normalizing these metrics by the size of study areas and the number of distinct drainage systems. Relationships between avulsion-frequency metrics and descriptors of process regime indicate that wave and tidal processes may stabilize coastal channel systems, but also that their influence may be modest. No consistent relationship is seen between avulsion-frequency proxies and the offshore bathymetric gradient, which in the studied examples does not scale with the rate of shoreline progradation. No evident trend exists between measures of avulsion frequency and estimated rates of either eustatic or RSL fluctuations. Overall, the considered variables do not leave a clear statistical signature in Holocene avulsion histories, suggesting that upstream or intrabasinal factors may represent more important controls.