TABLE 1-4 



WETLAND AREA AS A PERCENT OF TODAY'S ACREAGE FOR A 10- to 100-cm 

 RISE IN SEA LEVEL IN EXCESS OF VERTICAL ACCRETION* 



^Calculations are based on the assumption that development does not prevent new wetlands 

 from forming inland. If adjacent lowlands are protected, rises of between 1 and 1.6 m would 

 destroy the remaining marsh. 



Barrier Islands, Deltas, and Saltwater Intrusion 



Although most marshes could probably not keep pace with a substantial acceleration in sea 

 level rise, three possible exceptions are the marshes found in river deltas, tidal inlets, and on the 

 bay sides of barrier islands. River and tidal deltas receive much more sediment than wetlands 

 elsewhere; hence they might be able to keep pace with a more rapid rise in sea level. For 

 example, the sediment washing down the Mississippi river for a long time was more than enough 

 to sustain the delta and enable it to advance into the Gulf of Mexico, even though relative sea 

 level rise there is approximately one centimeter per year, due to subsidence (Gagliano, Meyer- 

 Arendt, and Wicker 1981). A global sea level rise of one centimeter per year would double the 

 rate of relative sea level rise there to two centimeters per year; thus, a given sediment supply could 

 not sustain as great an area of wetlands as before. It could, however, enable a substantial fraction 

 to keep pace with sea level rise. 



In response to sea level rise, barrier islands tend to migrate landward as storms wash sand 

 from the ocean side beach to the bay side marsh (Leatherman 1982). This "overwash" process 

 may enable barrier islands to keep pace with an accelerated rise in sea level. However, it is also 

 possible that accelerated sea level rise could cause these islands to disintegrate. In coastal 

 Louisiana, where rapid subsidence has resulted in a relative sea level rise of one centimeter per 

 year, barrier islands have broken up. The Ship Island of the early twentieth century is now known 

 as "Ship Shoal" (Pendland, Suter, and Maslow 1986). 



Marshes often form in the flood (inland) tidal deltas (shoals) that form in the inlets between 

 barrier islands. Because these deltas are in equilibrium with sea level, a rise in sea level would 

 tend to raise them as well, with sediment being supplied primarily from the adjacent islands. 



17 



