The shape and structure of deltas is influenced by the stickiness of their sediments, according to a paper online this week in Nature Geoscience. Sediment cohesion acts in conjunction with other processes, including river discharge, waves and tides.

Douglas Edmonds and Rudy Slingerland use numerical models to simulate the effect of sediment cohesion on the development of river deltas. Holding all other factors constant, they found that the relative stickiness of the sediments determined whether the deltas developed into bird's-foot deltas with complex floodplains, such as the Mississippi River Delta, or fan-shaped deltas with smooth shorelines, such the Nile River Delta.

Sediment cohesion is controlled in part by the amount and type of vegetation surrounding the river. Therefore, the authors suggest that before the widespread evolution of continental plant life ― beginning about 420 million years ago ― river deltas should have been predominantly fan-shaped, a finding that is consistent with the rock record.