Subsidence in coastal Louisiana: causes, rates, and effects on wetlands – 1983

Boesch, D. F., D. Levin, D. Nummedal, and K. Bowles. 1983. Subsidence in coastal Louisiana: causes, rates, and effects on wetlands. U.S. Fish and Wildlife Service, Division of Biological Services, Washington, DC. FWS/OBS-83/26. 30 pp.

Summary

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 emergent vegetation. Although global sea level has risen only slowly (about 1.5 mm/yr) during the past century, sea level may rise more rapidly due to warming induced by the buildup of C02 in the atmosphere. Currently, apparent sea level along coastal Louisiana as reflected in tide gauge records, has risen more than five times faster than global sea level due to subsidence of thick unconsolidated sediments.

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 I 0 mm/yr in wetlands near the gulf coast. Subsidence rates over the last I ,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.

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