channels and allow the intrusion of salt 

 water deep into the estuary. Saltwater 

 accelerates the conversion of fresh and 

 intermediate marshes to saline marshes. 

 When increases are sudden, sal t-intolerant 

 vegetation can be killed, and the marsh 

 may erode before other vegetation can be 

 established. There is also some 



suggestion that the biochemistry of marsh 

 sediments changes with salinity, making 

 the marsh more vulnerable to erosion 

 (Dozier 1983). 



A network of medium-sized canals that 

 are dredged for access to oil and gas well 

 sites is linking the navigation canals to 

 the inner marsh and to the flood drainage 

 canals. These canals are extensive; their 

 impacts are multiple. The canals 

 themselves act like the navigation canals 

 and, in combination with them, change 

 circulation patterns extensively. For 

 example, in the Leeville oilfield 

 (Terrebonne basin) the density of natural 

 channels declined as dredged channels 

 captured the flow of water (R. E. Turner, 

 LSU Center for Wetland Resources; pers. 

 comm.). These canals also allow salt 

 intrusion. Their spoil banks block the 

 flow of water across marshes, depriving 

 them of sediments and nutrients. This is 

 especially noticeable where canals 

 intersect and their spoil banks interlock 

 to impound or partially impound an area. 

 The effect has not been rigorously 

 quantified, but aerial photographs showing 

 the loss of marsh in these semi-impounded 

 areas are too striking to ignore. 



Analysis of marsh loss rates between 

 1955 and 1978 (mapped by Wicker 1980) 

 shows a direct linear relationship between 

 canal density and the marsh loss rate 

 (Turner et al . 1982). The rate of loss 

 per unit of canal is higher in recently 

 formed deltas where the sediments are less 

 consolidated than in older deltas (Oeegan 

 et al. 1983). It seems to be maximum 

 where fresh marshes are experiencing salt 

 intrusion (Dozier 1983). Turner et al. 

 (1982) found that the intercept of the 

 regression of marsh loss on canal density 

 (that is where canal density is zero) was 

 always less than 10 percent of the total 

 loss and usually nearly zero. This 



Table 4. Land-use changes along the 



northwest edge of the Barataria basin, on 



the Bayou Lafourche natural levee (Dozier 

 1983). 



a. Change in developed land 

 Year Developed Rate of 



land area increase 



1945 

 1955 

 1969 

 1980 



(ki) 



19.27 



20.80 



39.41 



71.69 



(km /yr) 



0.13 

 1.43 

 2.93 



b. Loss of marsh to indicated category, 

 1945-80 



Area Marsh loss 



indicates that nearly all the loss can be 

 attributed to canals. The direct impact 

 of canals (the area they occupy) is less 

 than 10 percent of the total loss. If the 

 spoil area is taken to be three to five 

 times the canal area (Johnson and 

 Gosselink 1982), the direct loss of marsh 

 due to canals is less than 50 percent of 

 the total loss. The rest is attributed to 

 indirect effects of circulation disruption 

 by the canal and its spoil. 



An independent, lesser source of 

 marsh loss is direct impoundment and 

 drainage for agriculture or other develop- 

 ment. Several large reclamation projects 

 were initiated early in the century. Most 

 of these were destroyed by floods like the 

 one in 1927 and now appear as large, 

 square lakes in the coastal zone. How- 

 ever, reclamation along the natural levees 

 is proceeding apace, as is shown for the 

 Bayou Lafourche levee on the northwestern 

 side of Barataria basin (Table 4). Over 

 the region as a whole, especially in the 

 urban areas, agricultural land has been 

 converted to urban and industrial use 

 without a large net reclamation of new 

 marsh (Table 5). 



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