inundated by rising sea levels in response to land subsidence and reduced sediment supply (Davis 

 1985). Net loss of wetland area in Louisiana has resulted despite rapid vertical accretion rates 

 (Hatton, Delaware, and Patrick 1983). Elsewhere, at various times in the recent past, expansion 

 of wetlands has occurred (e.g., Redfield 1972). 



Similarly, Johannesson (cited in Seliskar and Gallagher 1983) reports that the marsh 

 advanced seaward into an Oregon estuary at over 21 m per year during 1887-1939, but has since 

 slowed to about 0.15 m per year. However, in the current era such expansion appears restricted to 

 local sites. Important factors determining wetland response to sea level rise include the topogra- 

 phy of the wetland bottom, changes in upstream sediment supply and in growth rates of marsh 

 vegetation, and more recently, the presence of artificial structures such as sea walls. 



Salt marshes (saline wetiands) typically occupy zones bordering landward freshwater 

 environments and marine or brackish bays and estuaries, except in high-energy tidal areas 

 directly facing the open sea. However, under low energy conditions (e.g., the Florida panhandle), 

 marshes may front the open sea. All salt marshes are technically defined as vegetated saline 

 intertidal flats. Atlantic Gulf marshes originally covered about 2.02 x 10 6 ha (Davis 1985), and in 

 the United States as a whole, about 3 x 10 6 ha in 1922 (Teal and Teal 1969). Since then, U.S. 

 coastal wetlands were lost at a rate of 0.2 percent per year through 1954, and 0.5 percent per 

 year through 1974 (Gosselink and Bauman 1980). More recently, loss rates have diminished as a 

 consequence of protective legislation (Tiner 1984). 



Three major salt marsh groups are recognized in North America: (1) Bay of Fundy, New 

 England, (2) Atlantic-Gulf Coastal Plain, and (3) Pacific. Along the Pacific coast only 10-20 

 percent of the coastal area is suitable for marsh buildup because marsh development has been 

 limited by coastal uplift. In contrast, about 71 percent of the shoreline of the Atlantic and Gulf 

 coasts is associated with mud deposits in estuaries, lagoons, or salt marshes (Emery and Uchupi 

 1972). 



Salt marshes can be grouped into distinct vegetation zones determined by the extent of tidal 

 inundation (Figure 4-1). In the Atlantic and Gulf areas, low marsh zones subject to protracted 

 daily tidal flooding are dominated by Spartina altemiflora, except in subtropical latitudes. Along 



FIGURE 4-1 



CROSS-SECTION OF A TYPICAL NORTHEASTERN 

 ATLANTIC COAST SALT MARSH (from Tiner 1984) 



switchgrass 

 high-tide bush 



"•*■"-- T r -* 



black grass 



V 

 IRREGULARLY FLOODED MARSH 



salt hay cordgrass 



spikegrass 



salt marsh aster smoo,h cordgrass 



glasswort (< al1 '° rrn > 



smooth cordgrass 

 (short form) 



_L 



Spring or Storm Tide 

 Mean High Tide 



Mean Low Tide 



I 



REGULARLY 



FLOODED 



MARSH 



<7 



INTERTIDAL 

 FLAT 



V 



ESTUARINE 



OPEN 



WATER 



(BAY) 



88 



