52 



grained sediments may be supplied from marine, coastal cliff, fluvial, or 

 estuarine sources. 



Mudflat morphology is related to tidal and sediment processes, and the 

 break in slope at mid-tide generally reflects the position of the maximum tidal 

 velocity (Pethick 1984). The upper part of the mudflat surface progressively 

 increases in height due to the accretion of sediments, so that the period of 

 inundation of each tide is successively reduced. Eventually, the mudflat may 

 become exposed long enough to allow vegetation to colonize, leading to the 

 development of salt marshes. 



Salt marsh development usually begins with the deposition of mud on a 

 sand surface followed by the establishment of algae salt-tolerant plants like 

 Spartina Patens, Spartina Alternifora, and eelgrass near the high-water mark. 

 The vegetation and seaweed may assist in trapping increasing amounts of 

 sediment. Increased vegetative sediment trapping leads to the upward and 

 outward building of hummocks, and to reduced wave heights and energies. 

 Rates of marsh sedimentation are controlled by sediment availability and 

 vegetative trapping, and also by the magnitude and frequency of various 

 factors affecting water level elevation, including waves, tides, and surges. 

 With increasing elevation, however, there will be decreased frequency of 

 inundation, and thus reduced rates of upward accretion during storms. 



Salt marshes are well-developed during periods of relative sea level stabili- 

 ty, and typically occur on marine delta plains, behind barrier beaches, in 

 depressions, embayments, and other irregularities of the coast. Salt marshes 

 trap muddy sediments in low-energy tidal situations along protected sections of 

 extratropical coasts, comprising 1- to 2- km bands along the Atlantic and gulf 

 coasts of the United States. Numerous organisms are uniquely adapted to 

 such conditions, and the salt marsh substrate records contain many details that 

 are of significance in paleoecology and environmental reconstruction. 



Mangrove swamps occupy settings similar to salt marshes, except that they 

 occur in lower latitudes, between 30 deg N and 30 deg S. Mangroves include 

 several species of low trees and shrubs, and are characterized by an entangle- 

 ment of arching prop roots that facilitates trapping of fine sediment. 

 Typically, they are composed of higher amounts of organic debris than 

 marshes. Mangrove growth is favored by tidal submergence and high tidal 

 range, low coastal relief, saline or brackish water, abundant fine sediment 

 supply, and low wave energy. The most notable area of mangrove develop- 

 ment in the United States is in southwest Florida. 



Estuaries 



Estuary definitions vary, but all estuaries share several important attributes. 

 They are semi-enclosed water bodies in which tidal exchange and fresh water 



Chapter 3 Variable Coastal Features 



