Mexico, the canals flush fresher water from interior marshes and lower water levels. 

 Then, with high tides in the gulf, salt water is able to move farther inland. The process 

 is gradual, and a period of several years may be necessary for the effects to become 

 evident. 



As water salinity increases in an area, plants unable to tolerate the higher salinity 

 die and are gradually replaced by species adapted to the new salinity regimes. Greatest 

 damage to plants takes place when fresh marsh containing high levels of soil organic 

 matter is subjected to water of much greater salinity and strong tidal action. Plants in 

 the area are killed by increased water salinity, and the organic substrate becomes loose 

 and disorganized without plants roots to hold it together. As tide water moves through 

 the area, small amounts of organic matter are picked up by the current and flushed out 

 through tidal channels. Before new species can become established, marsh elevations 

 may drop 10 to 20 cm over broad areas. Open ponds and lakes thus develop and 

 productive marshland is lost. Thousands of hectares of marsh in the deltaic plain of 

 southeastern Louisiana have been thus affected. The chances of such areas again 

 supporting emergent plant growth is very unlikely unless corrective action is taken on a 

 large scale. 



Prior to levee construction along the Mississippi River, overbank flooding would 

 send vast quantities of fresh water and alluvium down former channels of the river and 

 other streams emptying into the gulf on the deltaic plain. In many areas flood water 

 from the Mississippi River would reach the Gulf of Mexico via sheet flow over the 

 marshes. 



As a result of overbank flooding, a tremendous area of fresh marsh was developed 

 and maintained. Also, nutrient-rich sediment was added to the marsh, thus enhancing 

 productivity and promoting land building. 



Because of several disastrous floods, the Mississippi River Commission was formed 

 in 1879, and levee construction for flood control began in 1882. Completion of the levee 

 system required many years, but today the levee system extends southward to the active 

 delta (approximately 100 km south of New Orleans). Approximately one-third of the 

 Mississippi River flow is diverted through the Atchafalaya River during flood stage. The 

 remainder is carried through the leveed channel of the Mississippi to the Gulf of Mexico. 



CHANGES IN VEGETATIVE TYPES 



A comparison of studies by Penfound and Hathaway (1939), O'Neil (1949) and 

 Chabreck (1970) disclosed that the plant species composition within vegetative types 

 changes very little over a period of several decades. Environmental conditions or 

 successional stages may cause certain species to become abundant locally. On a 

 coastwide basis, however, the species composition of individual types has remained 

 relatively stable. Changes most noticable were the decline of three-cornered grass 

 (Scirpus oineyi ) in the brackish type (Palmisano 1967) and sawgrass (Cladium jamaicense) 

 in the intermediate and fresh types (Valentine 1977). In recent years, smooth beggartick 

 (Bidens laevis) has greatly increased in the fresh vegetative type (Kinler et al. 1981). 



Although little modification has taken place within vegetative types, considerable 

 change has been noted among vegetative types during the past three decades. This 

 change was caused by coastal alteration which resulted in increased saltwater intrusion 

 and general shifts in the boundaries of vegetative types. 



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