the Pacific, several species are found, including Salicomia virginka, Spartina califomicus, and 

 Distichlis spicata. High marsh zones situated above daily high tides, but subject to spring and 

 storm tides, are dominated by Spartina patens and Distichlis spicata in the east, Juncus roemeri- 

 anus along the Gulf of Mexico, and by several species in the west, including Distichlis spicata, 

 Juncus balticus, and Deschampsia caespitosa. Landward of the saline marshes, brackish and 

 tidal freshwater marshes are found; these are particularly diverse and a number of subtypes have 

 been defined for both the Atlantic and Pacific coasts. They are typified by salinities below 0.5 ppt 

 and often can be distinguished from freshwater marshes found beyond tidal influence along the 

 Atlantic Coast (Odum and Fanning 1973). Tidal freshwater marshes are especially extensive in 

 Louisiana, which contains 210,000 ha, or 30 percent of the total marsh area of the Mississippi 

 Delta (Gosselink 1984). 



REGIONAL WETLAND DIFFERENCES RELEVANT 

 TO SEA LEVEL ADJUSTMENTS 



Tidal range, tidal regularity, and substrate type influence marsh boundaries in relation to a 

 specific tidal datum and therefore help determine adjustments to rising sea levels. Atlantic tides 

 are regular and nearly equal in semidiurnal range, whereas in the Pacific, tides exhibit a diurnal 

 inequality. Gulf Coast tides are irregular but of small amplitude; thus the distinction between high 

 and low marshes is less significant and the general marsh surface approximates mean high water. 

 In Massachusetts, however, the low marsh corresponds to the upper-middle intertidal zone 

 beginning between half-tide level and mean highwater neap. Along the Pacific coast, the low 

 marsh ends at the landward edge at about mean highwater neap. 



Regions also differ in their proportion of salt marsh types. Thus, New England marshes 

 consist mostly of high marsh meadow, with low marsh plants found mostly along tidal creek 

 borders (Miller and Egler 1950). South of Chesapeake Bay, low marshes increase in frequency. In 

 Georgia about 60 percent of the marsh area is stream side-levee marsh and low-marsh meadow 

 (Odum and Fanning 1973). 



Along the Gulf, however, irregularly flooded Juncus roemerianus marsh may predominate. In 

 southern California, marshes exhibit a conspicuous middle-marsh zone between low and high 

 zones. Despite the smaller marsh areas of the Pacific coast, its marsh floras are more diverse, 

 tidal ranges are greater, and the resulting zonation more complex. 



Northeastern Atlantic marshes commonly are dominated by brown or gray silt and clay 

 overlain by thin peat. In New England, because most glacially derived silts and clays have been 

 deposited in lakes and swamps or have been swept out to sea, less inorganic material remains 

 available for marsh deposition (Meade 1969). Instead, thick peat beds have accumulated (Redfield 

 1965, 1972) to depths as great as 59 m in offshore Pleistocene deposits. Inorganic sediments 

 often dominate sediments where glacial deposits have been reworked or coarse materials have 

 been ice-rafted to the marsh. Elsewhere in this region, however, organic material predominates in 

 marsh peat (Armentano and Woodwell 1975). 



South of Chesapeake Bay, peat substrates are relatively rare, except in Louisiana and 

 Florida. In California, thick peat layers are rare and sediments contain little carbon. In the 

 southeast, tidal flushing prevents peat accumulations as do rapid decay rates and slow rates of 

 coastal submergence. 



PAST SEA LEVEL RISE AND MARSH ACCRETION 



Although scientists differ as to rates of sea level rise, all agree that the Holocene Epoch has 

 been marked by a long-term trend of rising sea level (Figure 4-2). This transgression followed a 

 great lowering of sea level during the Pleistocene when cooling climate triggered the advance of 



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