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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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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 http://www.ces.clemson.edu/semaps/la/nogu-a.jpg
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 http://www.elknet.pl/gsi/galerie/nil-delta.jpg
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 http://rst.gsfc.nasa.gov/Sect17/BNGFLD5x5.jpg
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.
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 http://www.mvn.usace.army.mil/pao/bro/MRivDrainageBasin.gif
http://www.esl.lsu.edu/demos/modis/modis-diversions-highres.jpghttp://www.esl.lsu.edu/demos/modis/modis-diversions-highres.jpg
Mississippi Lobes Coleman, J. M. (1988). GSA Bulletin, v. 100, pp 999-1015.
New Orleans http://www.visit-new-orleans.com/visit-new-orleans-4.jpg http://www.globalsecurity.org/military/facility/images/new-orleans-fig12-1.gif http://www.victorsfinearts.com/images/newart/large/new%20orleans.jpg
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 999-1015.
Evolution of a Bay Fill Coleman, J. M. (1988). GSA Bulletin, v. 100, pp 999-1015.
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.
