Grain Size Variation, Discharge Rate, and Delta Island Formation

River deltas are magnets for human population, but their quickly evolving morphologies threaten inhabitants. To discuss how changing morphologies of river deltas may affect people, it is critical to understand natural processes influencing their evolution. Floods are important to the natural progradation and island formation of river deltas. Here we present results from laboratory experiments with incorporated floods that develop bifurcated and formed island deposits. This study focuses on the effects of varied grain sizes and discharge rates to island formation: 1) a deposit formed with a 50:50 mixture of ground walnut shell and quartz sand grains is compared to one developed either only walnut shell sediment or quartz sand, and 2) a deposit formed by a flow with a constant discharge is compared to one formed by a flow with varied discharges (with "heavy", more sediment-laden, floods and "light", less sediment-laden, interflood flows). Concerning grain size variation, the three experiments each developed unique shapes and bifurcations. The sand-only run developed an elongated shape of the deposit and bifurcations occurred close to the inlet that migrated upward. The ground-walnut-shell-only run developed a radial deposit and had a bifurcation far from the inlet that migrated downward. The run that used the mixture made an ovular-shaped deposit and a bifurcation was developed close to the inlet that migrated downward. Concerning discharge variation, the contrast between the amounts of sediment introduced during flood and interflood periods influenced deposit morphologies. Despite an equivalent volume of sediment and the same flood frequency across the runs, a deposit formed through "heavy" floods and "light" normal flows had a more radial shape than an ovular deposit formed through flows with constant discharge rates did. While the flow of the ovular deposit bifurcated clearly close to the inlet, the flow of the radial deposit did not bifurcate clearly, but formed an island on far from the inlet. Grain size and flood discharge influence bifurcation lengths of flows and the geometries of their islands. Floods are crucial events in the evolution of a river delta. Understanding the character of these floods that reshape the delta surface will be important to deciphering the blueprints of river deltas.