CONCLUSION 



Regional patterns of wetland distribution and the potential for loss or gain of wetlands from 

 sea level rise during the next century depend on two principal factors: (1) the tidal range within 

 which wetlands can occur and (2) the extent of the lowest Pleistocene terrace (often found at 

 approximately five feet in elevation above present sea level along tectonically stable coasts). 



Thus in New England, where there is virtually no low terrace, marshes occur in association 

 with pocket beaches in small coves and behind small sand spits. Although the tidal range is high 

 and thus favors maintenance of marshes, there is little lowland to be inundated and colonized by 

 marshes. Consequently, after 2075, when sea level rise exceeds the present spring high tide level, 

 present salt marshes will be lost with no compensating gain in new marsh area. 



In contrast, from Long Island to southern Florida, coastal slopes are gentle, barrier beaches 

 are common, and the low terrace is widespread. Tidal ranges are also moderately high. 

 Therefore, wetlands are an important component of the coastal system. Furthermore, in many 

 areas, unless development of resort communities precludes inundation of the low terrace, some 

 marshes will expand throughout much of the twenty-first century, decreasing only after the 

 protective beach ridges are breached. However, marshes will be lost in areas that have high 

 coastal dunes or that lack the low terrace. 



The Florida Keys and Everglades owe their existence to carbonate deposits that accumu- 

 lated in shallow water during higher stands of sea level in the Pleistocene. As the Keys are 

 inundated (in the absence of protective measures), a slight increase in mangrove swamps can be 

 anticipated; but after 2075 the region will rapidly become open water. The southern Everglades 

 will also disappear. 



The Gulf Coast is also a region of low slopes and barrier coastlines; but, unlike the Atlantic, 

 it has higher terraces along the coast and has very low tidal ranges. Therefore, the marshes are 

 more vulnerable to inundation and cannot migrate inland as readily as the marshes of the 

 Atlantic Coast. With few exceptions, the Gulf Coast marshes will gradually disappear until the 

 barrier islands are breached, at which time the marshes will decline precipitously. A notable 

 exception to this pattern is in the Mississippi Delta, where rapid subsidence is already overwhelm- 

 ing high sedimentation and accretion rates. In general, large-scale loss of marshes (far exceeding 

 the current rate) can be expected in this area early in the next century. 



Most of the West Coast is similar to New England: steep, rocky slopes predominate. 

 Wetlands are of minor extent but occupy a wide tidal range, so that they can be expected to 

 persist through most of the next century. The more extensive marshes in the tectonic lowlands of 

 San Francisco Bay and the Washington coast will probably expand onto adjacent lowlands unless 

 restricted by protective structures. 



Aggregating the individual case studies provides a convenient way to detect commonalities 

 in wetland response trends throughout the diverse U.S. regions. However, although the study 

 sites were chosen to achieve a representative sample of wetland types without a priori bias as to 

 expected responses, the case study sites were not randomly chosen nor was adequacy of sample 

 size assured. Therefore, the apparent patterns in any area cannot be interpreted as statistically 

 valid estimates of region-wide responses to sea level rise. Instead, the aggregated data are best 

 viewed as indicative of the class of responses likely to occur in coastal areas similar to the case 

 study areas. 



The percent change in wetiand area at each study site is given in Appendix 4-B. These 

 regional data have been summarized in Table 4-8, shown earlier. The aggregated data illustrate 

 the clear trend toward diminished wetlands in the next century as an overall response to 

 increased sea level rise (Table 4-8). 



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