wetlands are able to accrete more rapidly in the future, however, existing literature provides little 

 reason to believe that wetlands will generally be able to keep up with a one- or two-meter rise in 

 sea level. 



Tidal Flooding 



Because periodic flooding is the essential characteristic of salt marshes, increases in the 

 frequency and duration of floods can substantially alter these ecosystems. Salt marshes extend 

 seaward to roughly the elevation that is flooded at mean tide, and landward to roughly the area 

 that is flooded by spring tide (the highest astronomical tide every 15 days). Salt marsh plants are 

 different from most plants found inland in that they tolerate salt water to varying degrees (Teal 

 and Teal 1969). Coastal wetlands flooded once or twice daily support "low marsh" vegetation, 

 while areas flooded less frequentiy support high marsh species. Transition wetlands can be found 

 above the high marsh, in areas flooded less frequently than twice a month. 



The natural impact of a rising sea is to cause marsh systems to migrate upward and inland. 

 Sea level rise increases the frequency and/or duration of tidal flooding throughout a salt marsh. If 

 no inorganic sediment or peat is added to the marsh, the seaward portions become flooded so 

 much that marsh grass drowns and marsh soil erodes; portions of the high marsh become low 

 marsh; and upland areas immediately above the former spring tide level are flooded at spring 

 tide, becoming high marsh. If nearby rivers or floods supply additional sediment, sea level rise 

 slows the rate at which the marsh advances seaward. 



The net change in total marsh acreage depends on the slopes of the marsh and upland 

 areas. If the land has a constant slope throughout the marsh and upland, then the area lost to 

 marsh drowning will be equal to the area gained by the landward encroachment of spring high 

 tides. In most areas, however, the slope above the marsh is steeper than the marsh; so a rise in 

 sea level causes a net loss of marsh acreage. Two extreme examples are noteworthy: marshes 

 immediately below cliffs in New England and along the Pacific Coast could drown without being 

 replaced inland. In Louisiana, thousands of square miles of wetlands are within one meter of sea 

 level, with very narrow ridges in between and very little adjacent upland between one and two 

 meters above sea level. A one-meter rise in sea level could drown most of the wetlands there 

 without necessarily creating any significant new marsh (Louisiana Wetland Protection Panel, 

 1987; Gagliano et al. 1981). 



Figure 1-5 illustrates why there is so much more land at marsh elevation than just above the 

 marsh. Wetlands can grow upward fast enough to keep pace with the slow rise in sea level that 

 most areas have experienced in the recent past (Kaye and Barghoom 1964; Coleman and Smith 

 1964; Redfield 1967). Thus, areas that might have been covered with two or three meters of water 

 (or more) have wetlands instead (Figures 1-5A, 1-5B). If sea level rise accelerates only slightly, 

 marshes that are advancing today may have sufficient sediment to keep pace with sea level. But if 

 sea level rise accelerates to one centimeter per year (projected for 2025-2050), the sea will be 

 rising much more rapidly than the demonstrated ability of wetlands to grow upward in most areas 

 (Armentano et al., Chapter 4) and the increase in wetland acreage of the last few thousand years 

 will be negated (Figure 1-5C). If adjacent upland areas are developed, all the wetlands could be 

 lost (Figure 1-5D). 



An important factor in determining the vulnerability of marshes to sea level rise is the tidal 

 range, the difference in elevation between the mean high tide and mean low tide. Coastal 

 wetlands are generally less than one tidal range above mean sea level. 6 Thus, if the sea rose by 

 one tidal range overnight, all the existing wetlands in an area would drown. Tidal ranges vary 

 greatly throughout the United States. Along the open coast, it is over four meters in Maine, 

 somewhat less than two meters (about five feet) along the mid-Atlantic, and less than one meter 

 (about two feet) in the Gulf of Mexico (NOAA 1985). The shape of an embayment can amplify or 

 dampen the tidal range, however. Most notably, the estuaries behind barrier islands with widely 

 separated inlets can have tidal ranges of thirty centimeters (one foot) or less. The tidal range of 

 Chesapeake Bay is about fifty centimeters (NOAA 1985). 



11 



