Recently some permits for dredging in 

 the delta marshes have included require- 

 ments for marsh improvement elsewhere to 

 mitigate the damage in the permit area. 

 This is a creative mechanism for conserv- 

 ing marsh, although at the expense of 

 other marsh tracts. Unfortunately, the 

 methodology for assessing the true envi- 

 ronmental cost of canals is rudimentary, 

 so the relationship between the canal 

 damage and the mitigation effort is some- 

 wh a t a rb i t ra ry . 



If environmental costs of development 

 in wetlands are to be internalized by the 

 developer, we need much better information 

 about how to assess these costs. In a 

 recent article /Vnft et al . (in review) 

 present a methodology and make a bene- 

 fitrcost assessment of an oil well access 

 canal in the chenier plain. Based on 

 their methodology, they suggest that a 

 conservative estimate of the environmental 

 cost for a typical exploratory well is 

 $380,000 (1981 dollars) per kilometer of 

 access canal . 



common in the chenier plain than in the 

 delta, primarily because the firner 

 substrate in the cheniers makes levee 

 construction much less expensive and more 

 effective. 



The idea behind these impoundments is 

 to prevent salt intrusion and thus retard 

 marsh loss. Unfortunately, there is little 

 evidence to show that they are effective, 

 and some evidence to suggest that they are 

 not. Baumann, Conner, and Gosselink (LSU 

 Center for Wetland Resources; unpubl, MS.) 

 analyzed marsh loss rates in impoundments 

 compared to adjacent unimpounded areas, 

 and concluded that loss rates ivere 

 actually higher in impoundments than 

 outside them (Figure 70). Wicker et al, 

 (1983) also measured marsh loss rates in 

 different kinds of impoundments in the 

 Rockefeller Wildlife Refuge. Although 

 they presented no comparative data, it is 

 apparent from their maps that marsh degra- 

 dation is occurring in all the impound- 

 ments except perhaps those with pumps for 

 water level control . 



A word needs to be said about some 

 current practices that do not seen to 

 effectively retard marsh loss. One of 

 these is channelizing upland runoff. In 

 fairness, this practice is not used to 

 minimize marsh loss, but it is a common 

 flood control measure. The impact on 

 marshes is negative because it shunts the 

 sediments of rivers and runoff away from 

 marshes, both by leveeing rivers to 

 prevent overbank flooding and by digging 

 deep-dredged channels to deliver flood 

 water through and around marshes instead 

 of over them. This is a case of 

 conflicting interests in the coastal zone. 

 Until recently, flood control interests 

 took ascendancy over marsh loss concerns. 

 A more balanced evaluation of this 

 "solution" to flooding is needed. 



Another common practice is the 

 construction of levees and impoundments to 

 prevent marsh loss. In recent years, all 

 over the deltaic and the chenier plain 

 marshes small levees no nore than a meter 

 high have been thrown up by private land 

 owners. Marsh impound-nents are also 

 common in State and Federal wildlife 

 management areas where they were created 

 to improve habitat for waterfowl and fur 

 animals. These levees are much more 



The problem, I think, is that 

 sediment input is a key element in the 

 ability of a marsh to accrete fast enough 

 to keep up with subsidence. Impounding 



/f"-4 



z 

 111 

 a. ; 



o 



o 



NATimAL tUfiSH 



IMPOUNDED MARSH 



, K" 



1962 

 YEAR 



YEAR 



Figure 70. The increase in open water in 

 natural and impounded wetlands. The 

 pattern of greater wetland loss in 

 impoundments is consistent in both fall, 

 when water levels ire low, and winter, when 

 impoundments are flooded (W. Conner and R. 

 Baumann, Center for Wetland Resources, 

 Louisiana State University; pers. comm.). 



92 



