Chapter 4 



IMPACTS ON COASTAL WETLANDS 

 THROUGHOUT THE UNITED STATES 



by 



Thomas V. Armentano, Richard A. Park, C. Leslie Cloonan 



Holcomb Research Institute 



Butler University 



4600 Sunset Avenue 



Indianapolis, Indiana 46208 



INTRODUCTION 



Although wetland responses to sea level rise can be estimated only in association with 

 uncertainties inherent in making future projections, the major factors controlling wetland sea 

 level responses can be modeled. This chapter considers possible coastal wetland responses to 

 future sea level rise in the conterminous United States, in order to provide information needed to 

 understand future threats to coastal resources during an anticipated period of unprecedented 

 climatic change. 



Our primary objectives have been to interpret our present understanding of wetland adjust- 

 ments to sea level rise in terms of a future acceleration of present sea level rise rates, and to 

 outline a method for projecting future regional-level responses that could result from global 

 warming. The research focuses on relatively large-scale spatial patterns as opposed to specific site 

 responses. Therefore, local features often are subsumed within more widespread characteristics 

 in order to detect regional trends. 



SCOPE AND BACKGROUND 



The present study considers all coastal wetlands below 3.5 m elevation along the Atlantic, 

 Gulf, and Pacific coasts of the conterminous United States. Among the wetland types considered, 

 salt marshes predominate, although important brackish and freshwater marshes occur in each 

 coastal region. In subtropical Florida, mangrove swamps usually replace salt marshes. Although 

 all wetland types meeting the elevation criterion are considered, shifts between wetland types are 

 not explicitly treated, for reasons given later. 



The chief information base for this study consists of current knowledge of wetland adjust- 

 ments to sea level rise inferred for the past several thousand years, particularly during the present 

 century. The sedimentary sequence laid down under salt marsh conditions forms a record of 

 coastal history, thus providing a basis for dating the location of the intertidal zone at various 

 times in the past In many areas, reconstruction of past shorelines and of sediment profiles 

 reveals that the wetlands and sea level have been in approximate equilibrium for the past several 

 millennia. This condition appears typical of many Atiantic coast wetlands. However, the pattern is 

 not universal, and departures from this trend would be expected to influence wetland responses 

 to accelerated sea level rise. Thus, in recent decades, Texas and Louisiana wetlands have been 



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