abilities of wetlands, but data are not available to de- 

 termine the quantitative effects on water quality of 

 converting natural wetlands to impounded wetlands. 

 Judging by phosphorus loading, water-quality is mar- 

 ginal in the basin (table 3.69). 



There may be considerable potential for increased 

 commercial production of freshwater finfish species 

 and crayfish in the Mennentau Basin, since they do 

 not appear to be exploited to any great extent. The 

 supply of waterfowl for hunting exceeds the demand 

 in the mermentau Basin. 



The decline in wetland area can be expected to 

 lead to a decline of wildlife and water resources. Fur- 

 ther^ expansion of agriculture in the basin can occur 

 only at the expense ofnatural or impounded wetlands. 

 As agriculture expands, problems of advanced stages 

 of eutrophication will be compounded by increased 

 nutrients from fertilizer runoff, increased water use 

 for irrigation, and increased density of drainage canals. 



An infomial agreement among the U.S. Fish and 

 Wildlife Service (FWS), the Louisiana Department of 

 Wildhfe and Fisheries (LDWF), and the U.S. Army 

 Corps of Engineers (USAGE) was implemented during 

 the summer and fall of 1976. Under this agreement, 

 the timing and extent of drawdowns needed to en- 

 courage wildlife food-plant production and to partially 

 restore the use of the Memientau Basin by estuarine 

 organisms was attempted. Vaughn (Pers. Comm.) re- 

 ported an estimated inshore harvest of white shrimp 

 in excess of 160,000 kg (353,000 lb), and an offshore 

 harvest of 53,000 kg (1 1 7,000 lb), based on sampling 

 and surveys in White and Grand lakes. In addition 

 160,000 kg (353,000 lb) of blue crab were harvested. 

 The combined dockside value was estimated at 

 $1,321,000. A remarkable increase in annual grasses 

 and sedges was also reported. There seemed to be no 

 major conflict between this drawndown program and 

 rice culture, navigation, or flood control. 



3.6.4 CHENIER BASIN 



General features. The Chenier Basin is a long, 

 narrow, east-to-west strip of land and water sand- 

 wiched between the Mermentau Basin and the deep 

 Gulf waters. Well-developed chenier ridges along the 

 northern boundary, and control structures in opera- 

 tion since 1950 on the Mermentau River and Fresh- 

 water Bayou effectively cut this basin off hydrolog- 

 ically from the Mennentau and Vermilion basins (piate 

 IB). Beach ridges and smaller cheniers protect the in- 

 land area from direct Gulf influence (fig. 3-32). Fresh- 

 water input is limited to local rainfall and to the Mer- 

 mentau River discharge in the extreme western end of 

 the basin. The tidal action is strong and the natural 

 wetlands of the basin are all salt-influenced. Because 

 sediments from the Atchafalaya River drift westward, 

 mud flats are developing along the Vermilion Basin 

 coastline and are expected to develop westward across 

 the Chenier Basin. However, shoreline erosion is oc- 

 curring from Rollover Bayou to Hackberry Beach. 

 The beach is accretingslowly west of Hackberry Beach 

 to the Calcasieu River. 



Inland water bodies are few and cover only 2.9% 

 of the land area; most of them are associated with the 

 lower reaches of the Memientau River. Over two- 

 thirds of what was once natural marsh has been im- 

 pounded (25% of the total basin area). As a conse- 

 quence, natural circulation patterns through the basin 

 are severly modified. 



Nearly all hydrographic records are from the 

 lower Memientau River at Grand Chenier and at Cat- 

 fish Point control structure, both at the extreme 

 western end of the basin. 



Tidal range at Grand Chenier is 24 to 42 cm (9.4 

 to 16.5 in), but at Catfish Point this tide is completely 

 masked by the fiow through the control structure, 

 when it is open (fig. 3-41). The sudden release of large 

 volumes of fresh water also causes dramatic short-tenn 

 salinity decreases in the proximity of the control 

 structure (Perret et al. 1971). Long-temi mean water 

 levels at Grand Chenier show peaks in April and Sep- 

 tember, with little depression during the summer 

 months. Over the years, mean water level has been 

 rising (about 2.1 cm/yr or 0.83 in/yr) with respect to 

 the gage elevation at a rate comparable to the Mer- 

 mentau Basin (fig. 3-41). 



In addition to the Memientau River pass, a num- 

 ber of other ephemeral passes connect the inland por- 

 tion of the basin to the Gulf. These connections allow 

 a high diversity of estuarine-dependent fishes and 

 shellfishes in the inland water (Perret et al. 1971). 

 Shrimp and menhaden are the primary estuarine- 

 dependent commercial species caught in the basin. 

 Trapping of nutria and muskrat is an important indus- 

 try. Large populations of waterfowl and sport-fishes 

 are also found here. 



Socioeconomics. The population of the Chenier 

 Basin is scattered along the ridges. No dense popula- 

 tion centers nor manufacturing industries exist. Min- 

 eral extraction is virtually the only industry (table 

 3.70). The value of extracted oil and gas in 1974 was 

 $75 million, $0.7 million for agriculture, $1.5 million 

 for fishing and trapping, and an estimated $2 million 

 for sport fishing and hunting(Chenier and Mermentau 

 combined). 



Effects of Human Activities on the Environment. 



Hydrologic effects: Normal fiows of water in tiie 

 Chenier Basin have been modified by control struc- 

 tures and extensive impoundments (part 3.6.3). Man- 

 ipulation of the Catfish Point control structure 

 changes the volume and timing of discharge from the 

 Mennentau Basin into the Chenier Basin. In addition, 

 an extensive network of canals, 1,321 km (821 mi) in 

 length, covers 2% of the land area; and spoil banks 

 along these canals and along the lower Mennentau 

 River further restrict and modify drainage. 



Habitat effects . Since 1952, 33% of the natural 

 marshes have been lost. Four percent of the marshes 

 has been directly changed by canals— either to water 

 or to spoil. Over 900 ha (2,224 a) have been lost to 

 shoreline erosion by the Gulf. 



118 



