River-dominated deltas River-dominated deltas are often shaped like a bird’s foot. River-dominated deltas are dissected by numerous distributary channels. River-dominated deltas protrude far from the coast. Sediment in river-dominated deltas accumulates in shore-perpendicular ridges.

Mississippi Delta

Wave-dominated deltas Wave dominated deltas tend to be triangular in shape. Wave-dominated deltas do not have many distributary channels. Wave-dominated deltas do not protrude far from the coast. Sediment on wave-dominated deltas accumulates in shore-parallel ridges and bars.

Nile Delta

Tide-dominated deltas Tide-dominated deltas tend to be triangular. Tide-dominated deltas have poorly defined, interconnected distributary channels. Tide-dominated deltas do not extend far from coast. Sediment accumulates in shore-perpendicular linear islands and ridges.

Ganges Delta

Fraser River delta

Controls on Delta Plan Form Mode of mixing between inflow and basin waters Post-depositional processes

Mixing and Delta Form Mixing decelerates inflow. Deposition occurs where mixing occurs.

River-dominated Inflow © The Open University

River-dominated Deposition © The Open University

Wave-dominated Delta © The Open University

Tide-dominated Deltas © The Open University

Post-depositional Reworking Little post-depositional reworking occurs on river- dominated deltas because, by definition, waves and tides are weak. On wave-dominated coasts, longshore transport arising from wave refraction at mouth pushes sediment to the sides of river mouth. On tide-dominated deltas, sediment reworking forms linear islands.

Delta shape Birdsfoot deltas occurs on coasts with slow mixing of inflow and basin waters and limited depositional reworking. Triangular deltas are evidence of rapid mixing and extensive sediment reworking.

Distributary Channels Distributary channels are numerous where post-depositional reworking does not remove sediment from river mouth. Wave-dominated deltas therefore have few channels, but river-dominated deltas have many.

Delta Protrusion Deltas protrude far from the coast where post-depositional reworking is limited. Wave-dominated and tide-dominated deltas do not protrude from coast, and river- dominated deltas do.

Orientation of Sediment Bodies Waves produce shore-parallel bars. Currents favour shore-perpendicular deposition. Wave-dominated deltas have shore-parallel sediment bodies, and river- and tide- dominated deltas have shore-perpendicular bodies.

Mississippi Drainage Basin

Mississippi Lobes Coleman, J. M. (1988). GSA Bulletin, v. 100, pp

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Goals of River Control Correct, permanently locate and deepen the main channel Prevent floods

Mississippi Bay Fills Coleman, J. M. (1988). GSA Bulletin, v. 100, pp

Evolution of a Bay Fill Coleman, J. M. (1988). GSA Bulletin, v. 100, pp

River Control Problems Continued maintenance of main channel has led to significant accumulation of sediment at mouth. Water levels rise more dramatically during floods due to shallower gradient of river.

River Control Problems Levees must be built continually higher to protect against breaches and spillovers. Land adjacent to main channel is starved of sediment. Subsidence is causing severe coastal erosion.

Summary Deltas form where river-derived sediment accumulates faster than it can be dispersed. Delta profile depends strongly on the density contrast between inflow and basin waters.