Hydrologic manipulations are particularly prominent as the unintentional consequence of building 

 spoil banks during canal dredging. These spoil levees remain for several decades (Monte 1978). 

 Approximately 8% of the land surface within the deltaic plain from the Mississippi Sound to the 

 Atchafalaya River has been converted to canals and their associated spoil banks. This percentage 

 is equal to the best estimate of the density of natural drainage features. Canals and their spoil 

 banks and water management levees are therefore a major hydrologic factor influencing wetland 

 growth, maintenance, and decay. These levees will continue to be a management issue for the 

 lifetime of the levees. 



The direct environmental effect of canal construction-the conversion of marsh to open water 

 and spoil-is relatively easy to document. The indirect impacts, however, have only recently been 

 recognized and are, in general, poorly documented or understood (Allen and Hardy 1980). Such 

 effects are often related to the hydrologic regime of the wetland (King et al. 1982; Gilmore et al. 

 1981; Mendelssohn et al. 1982; Zucca 1982). Changes in wetland water circulation patterns have 

 been caused by long, straight, deep-dredged canals and by adjacent spoil banks which may block 

 normal overland flow and result in saltwater intrusion, acceleration of freshwater runoff, altered 

 sediment deposition patterns, and modification of nutrient supplies to adjacent wetlands. One 

 major indirect effect is to partially or completely impound marshes thereby reducing drying cycles 

 and increasing flooding times (Table 1). 



The consequences of increased flooding in salt and brackish marshes appear to be reduced marsh 

 plant vigor, accelerated subsidence rates, marsh breakup and loss, and decreased water quality 

 (Table 2) (Adkins and Bowman 1976; Craig et al. 1980; Turner 1987). The increase in canal 

 density exhibited during this century has been accompanied by an acceleration in annual land loss 

 rates and canal surface area (Scaife et al. 1983; Turner et al. 1983; Turner and Rao, in press). 



Table 1. Changes in hydrologic regime of a semi-impounded salt 

 marsh (from Swenson and Turner 1987). 



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