impoundments are used for water supply activities, the drawdowns can create 

 a broad devegetated zone around the edge which is essentially devoid of 

 life and subject to heavy erosion during periods of exposure. 



Dikes and levees should contain adequate engineering and management 

 provisions for release of fresh water to streams, floodplains, swamps, 

 and marshes in order to maintain favorable flow and overflow patterns to 

 protect fish and wildlife resources. 



Special care should be taken in evaluating the potential effects of 

 levee construction in marshes to ensure that the construction does not 

 lead to dessication of the marsh. Provisions must also be made so that 

 upon abandonment of an area, the levees or dikes are removed in order to 

 restore the area to its former condition [142]. 



4.16.2 Ditching 



Salt-marsh mosquitoes lay their eggs on damp soil under marsh or 

 mangrove vegetation above the mean high water level. Hatching and develop- 

 ment is triggered by periodic rainfall or tidal flooding. Reproduction 

 will not occur in the low-marsh areas flooded by daily tides [143]. 



To rapidly dry up the marsh surface and prevent hatching, the partial 

 draining of high marshes with grid-pattern ditching prevailed as a control 

 method through the 1940 's [144]. Such ditching is partially successful in 

 controlling mosquitoes but may be extremely damaging to the marsh ecosystem. 

 Ditching, using plows or trenching machines, creates a network of narrow 

 and shallow channels in the marsh. Dredge niaterial taken out of the 

 ditches typically is deposited on either side of the ditch. Ditching or 

 marshes has been found to be destructive to the existing vegetation and 

 wildlife of marshes. It is the general conclusion that ditching effective- 

 ly drains the marsh, converting low marsh into a community more 

 characteristic of the high marsh and uplands [145]. 



As the marsh dries and vegetative replacement takes place, drastic 

 modifications in the animal life of the marsh often occur, including 

 decreases of invertebrates, mostly molluscs and Crustacea. The cutting 

 of ditches also permits the intrusion of saline water into regions of the 

 marsh that were formerly brackish or relatively fresh. This adds to the 

 disturbance of the vegetation caused by lowering water tables. 

 Additional impacts are identified where spoil from ditches formed levees 

 which block tidal circulation [140]. 



4.16.3 Water Management 



Water management for pest control consists of an open-marsh water 

 management system of connected ponds and open channels developed to 

 provide access for mosquito-eating fishes and of impoundments created 

 by dikes to maintain stable water levels. 



175 



