The reasons for the high returns, however, are not 

 clear (see part 3.2.3). It is hkely that continued agri- 

 cultural expansion into marginal wetlands will acceler- 

 ate drainage canal construction and nutrient runoff. 

 Water quality is already a problem in the VermiUon 

 River. Loading rates of P to the basin place it in the 

 excessive eutrophic state, primarily because of runoff 

 from upstream agriculture (table 3.68). Agricultural 

 expansion, therefore, is hkely to aggravate eutrophica- 

 tion problems in the more enclosed bays and small 

 lakes. The Atchafalaya River undoubtedly influences 

 tlie productivity and diversity of Vennihon Bay but 

 its significance has not been totally considered. 



Effects on renewable resources: The overall trend 

 in habitats is for a continuous, slow conversion of na- 

 tural areas to culturally maintained systems. The con- 

 version of relatively unique swamp forest and ridge 

 habitats in particular, can be expected to result in 

 permanent loss of some of the rarer animal species 

 that hve in these habitats. Both habitats normally 

 support a diverse flora and fauna. Because they are 

 elevated areas in the middle of lands subject to inunda- 

 tion, ridges have a particularly valuable function dur- 

 ing storms and as a refuge for migratory song birds 

 (part 4.13) 



The silt-laden Atchafalaya waters are probably 

 the most important influence on the fishery resources 

 of the Vermilion Basin. The extensive oyster reefs 

 that once fringed the gulfward edges of the bay have 

 been smothered by sOt or killed by the freshwater. 

 The average salinities decreased to 3 %o in the bay 

 (Juneau 1975). Throughout the Atchafalaya and Ver- 

 milion bays, typical freshwater species such as white 

 crappie, bluegill, sheepshead minnow, and blue cat- 

 fish are found in the same waters as such marine 

 organisms as the Atlantic midshipman. Gulf toadfish, 

 Atlantic cutlassfish, and Atlantic stingray (Juneau 

 1975). As mudflats build out and become stabilized 

 over the shallow shelf, a diverse benthic fauna should 

 develop. This in turn should benefit demersal fishes. 



The Vemiilion Basin is severly impacted by activ- 

 ities associated with oil and gas recovery, and with 

 agriculture. These activities generally lead to acceler- 

 ated rates of wetland loss, and eutrophication is al- 

 ready evident. At the same time, rapid land accretion, 

 extreme turbidity, and high nutrient loads are result- 

 ing from the delta-buUding processes of the Atcha- 

 falaya River. Because of both cultural and natural 

 processes, this basin is an area of intense ecological 

 interest and worthy of wise management practices. 



3.6.3 MERMENTAU BASIN 



General features. The Mermentau Basin is unique 

 in the Chenier Plain for several reasons. It was formeriy 

 part of the Mermentau/Chenier drainage system, but 

 the natural chenier ridges along its southern boundary 

 and a number of water control structures have es- 

 sentially resulted in a single, large freshwater im- 

 poundment. Therefore, the basin has no nearshore 

 Gulf habitat. Several large shallow lakes cover about 

 one-quarter of the basin area (fig. 3-32). The natural 

 and impounded wetlands (47% of the area) are all 

 fresh. Most of the remaining land, which lies along 



the northern edge of the basin, is used for rice cultiva- 

 tion and for cattle. 



The basin is supplied with fresh water (fig. 3-37) 

 by the Mermentau River, which cuts diagonally across 

 it. Water control structures at Catfish Point, Schooner 

 Bayou and the Superior Canal, the Vermihon and Cal- 

 casieu locks on the GIWW, and locks at Freshwater 

 Bayou (fig. 3-32) restrict the fiow of fresh water out 

 of the basin and of salt water into the basin. The 

 main purpose of the control structures is to provide 

 a large freshwater reservoir for agricultural (rice) 

 irrigation so as to prevent tidal flooding, and to pro- 

 vide higher water levels for navigation. The locks and 

 control structures are manipulated to maintain mini- 

 mum water levels within the basin at 60 to 70 cm (24 

 to 28 in) above Gulf MLW and to prevent salt intru- 

 sion. They are generally closed on incoming tides and 

 when inside stages decline below 0.66 m (2.17 ft) 

 MLG. However, they are opened when stages exceed 

 0.60 to 0.67 m (1.97 to 2.20 ft) MLG and flows are 

 adequate to prevent salt intrusion. 



Before 1951, surface water in the basin was 

 pumped into rice fields and the flow in the Mermentau 

 River was often reversed. Upstream flows of up to 

 56.6 m^l sec (2000 ft^/sec) were recorded. This 

 caused saltwater intrusion into the lower basin (Army 

 Corps of Engineers 1961). Fresh water moves laterally 

 between the Mermentau and Calcasieu basins via the 

 Calcasieu Lock, depending on the direction of the hy- 

 draulic head. 



Because of these control structures, there is no 

 significant diurnal tide within the basin. Wind tides 

 dominate the circulation of Grand and White lakes. 

 Seasonal water levels are modified from the typical 

 dual spring and fall peaks. They are relatively high all 

 year except during June and July (fig. 3-38). The 

 water is neariy fresh year round, but chlorinity rises 

 to about 1.7 %o inside the Catfish Point control 

 structure in June and July (fig. 3-39). Since the con- 

 trol structures were installed in 1950, salinity appears 

 to have been declining slowly, although year-to-year 

 variability is high. Since 1965, rainfall has generally 

 been above normal, and it appears that of the control 

 structure has reduced salinities (fig. 3-38) under these 

 circumstances. Gages show a net long-term water level 

 increase of 2.13 cm(0.83 in) per year in the basin (fig. 

 3-38). 



Historically, the Mermentau was an estuarine 

 nursery ground, but it no longer functions in this 

 capacity as far as fisheries are concerned (Gunter and 

 Shell 1958, Morton 1973). Commercial freshwater 

 fishing for catfish and other species exists in Grand 

 Lake, White Lake, and adjacent waters. The major 

 commercial living resource is nutria (table 3.69). The 

 area attracts large numbers of waterfowl, both because 

 of the extensive fresh marshes and also because of the 

 nearby rice fields. The potential for freshwater fin- 

 fishing is also high, although there is very httle re- 

 corded commercial use of this resource. 



Socioeconomics. Most of the residents of the 

 Mermentau Basin are members of farming families. 



105 



