Subsidence in coastal Louisiana: Causes, rates, and effects on wetlands
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Coastal wetlands are being lost at a rapid and accelerating rate in Louisiana. Much of this loss is attributable to a relative lowering of the wetland surface below the level adequate to support vegetation. Such rapid subsidence is a natural phenomenon, related to the progradation and abandonment of distributary lobes of the Mississippi River Deltaic Plain. For a considerable period after abandonment of new sediment sources, wetlands are able to keep pace with subsidence by accreting sediments reworked by marine processes. But inundation of interior wetlands removed from such an active sediment supply, wave exposure, and saltwater intrusion eventually result in deterioration of the wetlands. Human activities may accelerate this process by disrupting sediment supplies for wetland accretion, raising global sea level, causing saltwater intrusion, and withdrawals of subsurface materials. Present subsidence rates from tide gauge records exceed 40 mm/yr at the modern Mississippi River delta and approximate 10 mm/yr in wetlands near the gulf coast. Subsidence rates over the last 1,000 years appear to have been half the rates presently observed. This either results from natural variability, inaccuracy of tide gauge records, or human influences. The effect of the high rate of increase in locally apparent sea level on wetlands is difficult to quantitatively predict because of local variations in subsidence and accretion, uncertainty regarding future global sea level, and lack of knowledge of the accretionary limits of wetlands.