30_ 



E 

 u 



g 



a 



a 

 ■o 

 c 

 o 



m 



15 







E 

 u 



CO 



< 



45 



30 



a 



^ 15_ 



o 



Inside North Control Gate, 



_L 



± 



M 



M 



J J 



Month 







N 



Figure 3-40. Monthly mean water levels above mean sea level (MSL) inside and outside the Freshwater Bayou 

 control structure (1963-1974), from U.S. Army Corps of Engineers gages. 



In addition to loss of natural marsh, there has 

 been a small loss of upland forest and swamp forest 

 habitats, primarily to agricultural use. These habitats 

 each compose less than 1% of the basin area. Swamp 

 forest habitat, in particular, is rapidly disappearing 

 from the Chenier Plain region. 



Effects on renewable resources: In a Study con- 

 ducted from September 1951 through June 1953, 

 immediately after installation of the locks and con- 

 trol structures, Gunter and Shell (1958) found that 

 marine and estuarine organisms dominated Grand 

 Lake, and nearby Little Bay. Morton (1973) has shown 

 that the basin now functions as a fresh-water im- 

 poundment and 80% of the aquatic species are those 

 typical of freshwater areas. Erosion of the shorelines 

 of White and Grand lakes has been extensive and 

 severe. Nutria, the most important furbearer, appears 

 to be declining because of the high water levels, and 



muskrats have not occurred in the basin since installa- 

 tion of the control structures. 



The results of impounding wetlands may have 

 long-term implications for water quality management. 

 The impoundment of wetlands within the Mermentau 

 Basin may have different implications than elsewhere. 

 Because of water control structures, the basin does 

 not function effectively as an estuarine nursery. The 

 marshes within the impounded basin do function ef- 

 fectively as habitats for waterfowl, nutria, alligators, 

 wading birds, and other marsh wildlife. The impound- 

 ments may have long-term implications for water qual- 

 ity because the basin has a relatively long freshwater 

 flushing time (83 days, table 3.69). Heavy loads of 

 agricultural fertilizer are drained from the rice fields, 

 and the drainage network of canals accelerates the 

 runoff process and bypasses normal overland flow, 

 dumping nutrients directly into the shallow lakes. Im- 

 poundments do not have the nutrient filtering cap- 



113 



