Significant effect of sediment cohesion on delta morphology

The morphologies of the world's deltas are thought to be determined by river discharge, tidal range and wave action(1). More recently, sea-level rise(2,3) and human engineering(4) have been shown to shape delta evolution. The effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary(4-6). In particular, the role of sediment cohesion, which is controlled by sediment size and type of vegetation, is unclear. Here we use a numerical flow and transport model(7-10) to show that sediment cohesiveness also strongly influences the morphology of deltas. We find that, holding all other factors constant, highly cohesive sediments form bird's-foot deltas with rugose shorelines and highly complex floodplains, whereas less cohesive sediments result in fan-like deltas with smooth shorelines and flat floodplains. In our simulations, sediment cohesiveness also controls the number of channels that form within the deltas, and the average angle of bifurcation of those channels. As vegetation generally acts as a cohesive agent, we suggest that deltas that formed before the expansion of land plants in the Devonian period should show fan-like characteristics, a finding consistent with the limited data from the sedimentological record(11).