Wetland Formation

wetland formation
Image and information from the Coast 2050 report



barrier islands
Barrier islands off the Louisiana coast
Louisiana Coastal wetlands began forming approximately 8000 years ago from deposition of sediments carried down the Mississippi River.  The Louisiana Coastal Wetlands consist of two connected systems known as the Mississippi deltaic plain and the Chenier Plain to the west.  The Louisiana Coastal Wetlands are part of a natural deltaic cycle of construction and deterioration.  The construction cycle lasted approximately 5000 years during relatively stable sea levels.  The majority of delta building took place at the mouth of the Mississippi, but subdeltas formed at the mouth of numerous other major rivers such as the Atchafalaya River and Wax Lake outlet.  Deltaic formation begins as sediment laden river water reaches the ocean and slows down causing much of the sediment to drop out of suspension.  Over time this accumulation of sediment leads to the formation of bars and shoals that further divert the water, causing increased deposition.  Flooding causes increased sediment loss, leading to the formation bars and shoals extending above sea level.  This newly-formed land is colonized with vegetation, which leads to further deposition and speeds up the land-growing process.  Floating mats of vegetation form causing the water flow to slow and drainage becomes sluggish.  The large amounts of vegetation and low flow cause anaerobic conditions to form. 

Regressive phase:  Delta building continues outward, causing an increase in land.  The maximum extent of the Louisiana Coastal Wetlands was approximately 4.5 million acres in the 1700’s. 

Deterioration phase:  Sediment deposition leads to the closure of distributary feeder channels causing the flow of freshwater to no longer reach the seaward edge of the wetland.  Subsidence causes much of the outer wetland to rapidly disappear.  Subsidence and wave action result in the formation of barrier islands.  Behind the barrier islands the masses of floating vegetation g ive way to ponds and eventually bays as salinity increases. 

Eventually the distributaries reopen and the formation cycle begins anew.  They cycle typically has an deterioration interval of approximately 5000 years.

 

 Pictoral progression of wetland formation
delta
Delta deposition and mudstream

fringe marsh
Fringe saline wetland
floating veg
Distributaries and floating vegetation

trees
Fresh water upland marsh


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Wetland Formation
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