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Wetlands account for most of the land less than 1 m above sea level. 

 These extensive marshes, swamps, and mangrove forests fringe most of the 

 U.S. coastline, particularly along the Atlantic and Gulf coasts. Their 

 estimated original extent in the United States was about 2.0 x 10^ km^ 

 (Hoese, 1967). This area has been significantly reduced through a variety 

 of actions, including an early widespread practice of filling marshlands in 

 urban areas. Wetlands loss has also been caused by other human actions, 

 such as the construction of canals and waterways and the diversion of 

 fluvial sediment to the offshore (National Research Council, 1987b). 



Ecological conditions in coastal marshes range from marine to nearly 

 terrestrial. A change in controlling factors, such as water salinity or 

 tidal and wave energy, will cause a displacement in marsh zonation. 

 Generally, coastal marshes are divided into low and high marsh based on 

 their elevation relative to sea level (Redfield, 1972) . Since marsh plants 

 are attuned to particular mean water levels, a rise in sea level will shift 

 the distribution of plant species proportionally landward. Beyond this 

 response to variation in relative sea level, however, a more complex set of 

 responses may occur, tied to the type of marsh considered. Thus, 

 anticipated changes in coastal marshes must be assessed within the context 

 of the basic marsh types that characterize the coasts. With respect to the 

 future effects of a rise in sea level, coastal marshes may be broadly 

 divided into backbarrier marshes, estuarine (brackish) marshes, and tidal 

 freshwater marshes (National Research Council, 1987b). 



Backbarrier marshes occur along the bayward sides of barrier systems 

 of the Atlantic and Gulf coasts. Studies (e.g., Zaremba and Leatherman, 

 1986) show that these marshes are formed and destroyed rapidly in such 

 dynamic environments . Maintenance of these marshes therefore appears to be 

 more a function of barrier stability than of the pace of upward growth of 

 the marsh surface, since sediment supplies are ample (Letzch and Frey, 

 1980) . For barriers rapidly migrating landward, there may be a net decline 

 in backbarrier marshes . This has been found to be the case at Assateague 

 Island, Maryland, where sediment blockage by jetties has greatly increased 

 the rate of landward barrier migration (Leatherman, 1983), and the same 

 qualitative result could be anticipated as a result of accelerated sea 

 level rise (National Research Council, 1987b). 



