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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.
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Mississippi Delta http://www.ces.clemson.edu/semaps/la/nogu-a.jpg
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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.
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Nile Delta http://www.elknet.pl/gsi/galerie/nil-delta.jpg
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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.
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Ganges Delta http://rst.gsfc.nasa.gov/Sect17/BNGFLD5x5.jpg
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Fraser River delta
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Controls on Delta Plan Form Mode of mixing between inflow and basin waters Post-depositional processes
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Mixing and Delta Form Mixing decelerates inflow. Deposition occurs where mixing occurs.
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River-dominated Inflow © The Open University
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River-dominated Deposition © The Open University
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Wave-dominated Delta © The Open University
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Tide-dominated Deltas © The Open University
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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.
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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.
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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.
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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.
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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.
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Mississippi Drainage Basin http://www.mvn.usace.army.mil/pao/bro/MRivDrainageBasin.gif
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http://www.esl.lsu.edu/demos/modis/modis-diversions-highres.jpg
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Mississippi Lobes Coleman, J. M. (1988). GSA Bulletin, v. 100, pp 999-1015.
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New Orleans http://www.visit-new- orleans.com/visit-new- orleans-4.jpg http://www.globalsecurity.org/military/fa cility/images/new-orleans-fig12-1.gif http://www.victorsfinear ts.com/images/newart/la rge/new%20orleans.jpg
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Goals of River Control Correct, permanently locate and deepen the main channel Prevent floods
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http://www.mvn.usace.army.mil/pao/bro/mrc_map.gif
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Mississippi Bay Fills Coleman, J. M. (1988). GSA Bulletin, v. 100, pp 999-1015.
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Evolution of a Bay Fill Coleman, J. M. (1988). GSA Bulletin, v. 100, pp 999-1015.
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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.
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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.
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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.
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