and alteration of circulation patterns may unbalance the ecosystem. 

 Such effects can be avoided by using natural channels and by minimizing 

 channel depths. 



Dimensions : Normally, the deeper and wider a channel is dredged, 

 the greater are the ecological effects, particularly in shallow waters. 

 Channels dredged into shallow-water areas should be limited to the 

 minimum depth needed for the majority of watercraft which will use the 

 area on a sustained basis. This practice lowers the amount and cost of 

 initial dredging and decreases the frequency of maintenance dredging 

 operations. 



Adverse effects are associated with the creation of areas too deep 

 to permit the light penetration necessary to sustain a balanced aquatic 

 habitat. Conversely, too shallow a depth causes excess turbidity, 

 caused by resuspension of silt by the prop wash from passing boats. 



Adverse effects can be reduced by minimizing the length, width, 

 and depth of the channels. In general, a navigation channel width 

 should not exceed three or four times the width of the largest vessel 

 for which it is designed. Similarly, operable depth should not exceed 

 3 feet (0.9 meter) deeper than the deepest draft vessel at low water. 

 Additional depth may be considered for newly dredged channels to 

 accommodate siltation or slumping, reducing shoaling and therefore, the 

 frequency of maintenance dredging. 



Deep harbor entrance channels may increase the inward flow of salt 

 water, accelerate sedimentation, and increase the inward transport of 

 materials from down harbor or from the ocean, as exemplified by the 

 problems of Savannah Harbor (Georgia) where channel deepening resulted 

 in increasing deposits of ocean source sediment [31]. 



Additional problems are created by imbalances in water circulation 

 resulting from major channel deepening, as demonstrated in the 

 Sacramento River delta project which would have greatly increased 

 the subsurface flow of salt water resulting in saltwater intrusion. 

 Such a change could shift and disrupt the salinity-dependent nursery 

 areas of striped bass and other important species and have other complex 

 effects. 



Depressions or "deep holes", resulting in long-term changes in 

 currents and water circulation, may affect mixing and flushing of 

 estuarine waters, eventually causing changes in water temperature, 

 salinity, dissolved oxygen, sediment accumulation, and ultimately low 

 net productivity [32]. Dredge holes may accumulate anaerobic sedi- 

 ments and pollutants which may leach into nearby water systems [32]. 

 Level -grade dredging and avoidance of channels deeper than the har- 

 bor sill depths reduce these impacts on estuarine resources. 



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