Distributary channels and their impact on sediment dispersal

A global analysis of world deltas, with details from a natural and an anthropogenic-influenced delta, demonstrates how distributary channels control the flux of sediment into the coastal ocean. The study addresses the range in the number of distributary channels across world deltas using remote sensing techniques. A power law relationship is found between the number of distributary channels and the length of river and, separately, the delta gradient. These relationships hold for all types of deltas whether controlled or strongly influenced by waves, river discharge, tides or ice (permafrost or sea-ice). Nature-controlled deltas, such as the Klinaklini delta, have distributary channels that act as overflow conduits that become active during flooding events. Anthropogenic-controlled deltas, like the Po delta, have distributary channels that are controlled for flood mitigation or low flow maintenance. Anthropogenic influences greatly impact the natural rate of delta progradation through changes in sediment supply, controlling the position of distributary channels, and impacting subsidence from gas and groundwater extraction. Even with flood controls, the Po delta traps 16% of the sediment load in its channels that are becoming super-elevated at rates of 4 to 10 cm/yr, with respect to the delta plain. A new model is formulated and shown to predict accurately the sediment flux through each channel, along with their hydraulic properties. Deltas with high numbers of distributary channels produce hypopycnal plumes with reduced transport capacity. As a result, sediment diffuses out of the multi-channel deltas as a buoyantly driven plume, rather than as a momentum driven jet.