dictates against this practice for 

 purposes . 



;iiost 



The impact of mineral extraction, 

 flood control, and navigation on marsh 

 loss occurs primarily through the canals 

 dredged for these operations. Table 34 

 lists the major ecological effects of 

 canals in the deltaic marshes, the kinds 

 of iiiechanisns that should minimize these 

 ecological impacts, and the specific 

 management practices that are being used 

 or could be used to implement these 

 mechanisms. Because good experimental 

 evidence is often lacking, many of the 

 effects and mitigation procedures are 

 inferred. I will document those 



statanents that can be documented. But 

 many are merely reasonable extrapolations 

 from what is known. 



Canals alter marshes by accelerating 

 salt intrusion, changing hydrology, and 

 affecting benthic and aquatic organisms. 

 Salt intrusion is closely tied to changes 

 in hydrology. It occurs when deep, 

 straight channels connect low-salinity 

 areas to high-salinity zones. Large 

 navigation channels that link the marshes 

 directly to the gulf are particularly 

 efficient in allowing salt intrusion 

 (Gosselink et al . 1979), but a channel 

 from a saline bay into a less saline marsh 

 also allows salt intrusion. 



Salt intrusion into fresh and 

 i ntennediate marshes stresses the 



vegetation. We do not know exactly how 

 the fairly subtle changes in salinity 



operate, but the result is often death of 

 the plants and, as the roots die, loss of 

 their peat-binding capacity. If the 

 salinity changes so rapidly that the 

 plants are not replaced immediately by 

 more salt-tolerant species, often the 

 underlying peat rapidly erodes and large, 

 shallow lakes appear (Dozier 1933). These 

 changes are linked to biocheinical and 

 microbial changes in the peat associated 

 with salt intrusion (Dozier 1983). 



Canals also change hydrologic 

 patterns that inodify a marsh independently 

 of any salt effect. Straight, deep canals 

 in shallow bays, lakes, and marshes 

 capture flow, depriving the natural 

 channels of water (L. Gosselink 1934; 

 Turner, pers. comm.). Canals are 



hydrological ly efficient, allowing more 

 rapid runoff of fresh water than the 

 normal sinuous channels. As a result, 

 water levels fluctuate more rapidly than 

 in unmodified marshes, and minimum levels 

 are lowered (Light 1976). Sheet flow of 

 water across the marsh surface is reduced 

 by the spoil banks that almost always line 

 a canal. Consequently, the sediment supply 

 to the marsh is reduced, and the water on 

 the marsh is more likely to stagnate than 

 when freely flooded. 



Since canals change the marsh water 

 budget, the salt budget, and the sediment 

 supply, any mechanisms that can influence 

 these three factors might be useful ways 

 of minimizing the effects of canals. 

 Table 34 lists several mechanisms. 

 Generally, an increased freshwater supply 



Table 34. Impacts of canals in Louisiana coastal marshes leading to habitat loss, 

 and mechanisms and management practices to minimize these impacts. 



Type of impact 



Mechanisms to minimize impacts 



Management practices 



Increase fresh water supply 1. 



Increase sediment supply 2. 



Reduce salt intrusion 3. 



Maintain slow, sinuous natural 4. 



water flows 



Maintain overland flow 5. 



Maintain water levels 6. 



Fresh water diversion 

 Reduce number of canals 

 Control canal location 

 Improve engineering 



design 

 Backfill canals 

 Require mitigation 



fee for lost resources 



90 



