Once leveed and drained, the wetland environment undergoes subsidence 

 resulting from drying .compaction and oxidation, which may lower the ele- 

 vation as much as a foot per year during the first few years. After 

 initial subsidence, a slower but continual subsidence is evidenced as 

 organic matter in the soil decomposes [142], 



About 10,000 acres of tidelands and wetlands diked in southern New 

 Jersey by salt hay farmers became mosquito havens and required the appli- 

 cation of pesticides. It is believed that if the diked lands were re- 

 stored to natural marsh by removing the dikes, 20,000 clapper rails would 

 reside there, and 10,000 black ducks would winter there [150]. 



Estuarine waters are affected by the changes in water circulation, 

 salinity balance, sediment transport, flushing rate, and natural supply 

 of nutrients from upstream. Such changes affect the physical makeup of 

 the estuarine zone, its energy flows and the plant and animal life, and 

 can create many practical problems. There are examples in Louisiana 

 where fresh water supplies cut off by levees must be replaced by pump- 

 ing to dilute the salt water sufficiently to discourage marine predators, 

 particularly oyster drills [164]. 



The presence of dike or levee barriers in "open water" areas across 

 former closely interdependent natural estuarine communities causes long- 

 term changes in migratory fish and shellfish breeding and feeding access, 

 in water flow, and in the basic food web structure and productivity charac- 

 teristics of the ecosystem. 



4.17.3 Impoundment 



Freshwater impoundments are often created for waterfowl habitat 

 management and mosquito control by diking. To be effective, che dike 

 system must be continuous and relatively impermeable. Tide gates or 

 weirs across channels may be used as control structures to permit high 

 water overflow and required drawdown. For example, opening a mosquito 

 impoundment to normal water flow after the mosquito breeding period is 

 over is recommended practice in many cases [153]. 



In addition to the problems created by the dikes themselves, 

 discussed in the previous subsection, impoundments preempt the existing 

 coastal wetlands habitat and convert it to an entirely different type of 

 biological system. The new system obviously favors the purpose for 

 which it is managed. 



" As ari"exampTe "of "befiefTts, waterfowl and wadfng Birds fF marsF 

 areas in Delaware increased tremendously after impoundment. Breeding 

 birds increased in numbers over the unmanaged, natural marsh. Areas of 

 open water with emergent vegetation created by the impoundment were found 

 to be optimum for most ducks and geese. Increased fish populations 



18; 



