Moreover, if sea level rise causes barrier islands to breach, additional tidal deltas will form in the 

 new inlets, creating more marsh, at least temporarily. In the long run, however, the breakup of 

 barrier islands would result in a loss of marsh. Larger waves would strike the wetlands that form 

 in tidal deltas and in estuaries behind barrier islands. Wave erosion of marshes could also be 

 exacerbated if sea level rise deepens the estuaries. This deepening would allow ocean waves to 

 retain more energy and larger waves to form in bays. Major landowners and the government of 

 Terrebonne Parish, Louisiana, consider this possibility a serious threat and are taking action to 

 prevent the breakup of Isle Demiere and others around Terrebonne Bay (Terrebonne Parish 

 1984). 



Sea level rise could also disrupt coastal wetlands by a mechanism known as saltwater 

 intrusion, particularly in Louisiana and Florida. In many areas the zonation of wetlands depends 

 not so much on elevation as on proximity to the sea, which determines salinity. The most 

 seaward wetlands are salt marshes or their tropical equivalent, mangrove swamps. As one moves 

 inland, the fresh water flowing to the sea reduces salinity, and brackish wetlands are found. Still 

 farther inland, the freshwater flow completely repels all salt water, and fresh marshes and cypress 

 swamps are found. 



Although these marshes may be tens (and in Louisiana, hundreds) of kilometers inland, their 

 elevation is often the same as that of the saline wetlands. A rise in sea level enables salt water to 

 penetrate upstream and inland, particularly during droughts. In many areas, the major impact 

 would be to replace freshwater species with salt-tolerant marsh. However, many of the extensive 

 cypress swamps in Louisiana, Florida, and South Carolina, as well as some "floating marshes," 

 lack a suitable base for salt marshes to form. These swamps could convert to open water if 

 invaded by salt, which is already occurring in Louisiana (Wicker et al. 1980). 



HUMAN INTERFERENCE WITH NATURE'S RESPONSE TO 



SEA LEVEL RISE 



Although the natural impact of the projected rise in sea level is likely to reduce wetland 

 acreages, the ecosystems would not necessarily be completely destroyed. However, human 

 activities such as development and river flow management could disable many of the natural 

 mechanisms that allow wetlands to adapt to a rising sea, and thereby substantially increase the 

 loss of wetlands over what would occur naturally. In some areas the impacts could be so severe 

 that entire ecosystems could be lost. 



Development and Bulkheads 



Although environmental regulations have often prevented or discouraged people from 

 building on wetlands, they have not prevented people from building just inland of the marsh. As 

 the final box in Figure 1-5 shows, wetlands could be completely squeezed between an advancing 

 sea and bulkheads erected to protect developed areas from the sea. A few jurisdictions, such as 

 Massachusetts, currently prohibit additional construction of bulkheads that prevent inland 

 advance of marshes. 9 However, these provisions were enacted before there was a concern about 

 accelerated sea level rise; it is unclear whether they would be enforced if sea level rise accelerates. 

 Moreover, bulkheads are already found along much of the shore and are generally exempt from 

 such provisions. 



The amount of sea level rise necessary for development to prevent new marsh from forming 

 would depend on the extent to which development is set back from the wetlands. In Maryland, for 

 example, the Chesapeake Bay Critical Areas Act forbids most new development within 1,000 feet 

 of the marsh; thus, if the sea rises 50 centimeters (the highest part of the marsh) in excess of the 

 vertical accretion, there may still be 1,000 feet of marsh. Additional rises in sea level, however, 

 would eventually squeeze out the marsh. 



18 



