Image and information from the Coast 2050 report
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
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.
Delta deposition and mudstream
Fringe saline wetland
Distributaries and floating vegetation
Fresh water upland marsh