standard Deviation 



1 _ 



o 



♦<• 

 c 



o 



O 



2_ 



J F 



AMJJASOND 

 Month 



Figure 3-39. Monthly means and standard deviations 

 (1947 to 1974) of chlorinity in the 

 Mermentau Basin inside Catfish Point 

 control structure, from U.S. Army Corps 

 of Engineers records. 



The few communities along the northern boundary of 

 the basin are agriculturally oriented. These people are 

 not adequately represented in the employment statis- 

 tics in figure 3-12, which report only employees cov- 

 ered by the Federal Insurance Compensation Act. 



Despite the importance of farming, the largest in- 

 dustry in the basin is mineral extraction. Minerals, 

 the most valuable products of the basin, were worth 

 $1 14 million in 1974. Agricultural products are worth 

 about $14 milhon per year; commercial fishing and 

 trapping, about $1 million;and sport fishing and hunt- 

 ing, $2 million. 



The volume of waterborne commerce into and 

 through the basin is stable at about 2 miUion tons 



(1.8 million tonnes) per year, most of it (1.4 million 

 tons or 1.27 million tonnes) involves the export of 

 crude petroleum. This volume is small compared to 

 the 50 million tons (45 million tonnes) of traffic in 

 the Calcasieu Basin and 100 miUion tons (91 miUion 

 tonnes) in the Sabine area. 



Effects of Human Activities on the Environment. 



Hydrologic effects : The extensive modification of the 

 natural hydrologic regime of the Mermentau Basin by 

 control structureshas been described. Within the basin, 

 circulation patterns have undoubtedly been modified 

 by the extensive canal network 2,826 km (1,756 mi) 

 long (plate 5B), covering 2.1% of the basin area (table 

 3.69). In addition, impounded wetlands within the 

 basin cover 18% of the area and further modify the 

 inundation and flushing patterns of the wetlands. A 

 final factor is the withdrawal of fresh water for rice 

 irrigation. This is calculated at 3 x 10* m'' (1.1 x 10'° 

 ft^), about one-third of the total flow of the Mer- 

 mentau River (table 3.69), the major stream feeding 

 the basin, and about 10% of the total annual water 

 surplus of the basin, including rain. This demand oc- 

 curs almost entirely during April, May, June, and July 

 when the basin nomially sustains rainfall deficits (fig. 

 3-37) and river discharge is at its minimum. About 

 40% of the irrigation water is returned to the basin 

 when rice fields are drained toward the end of sum- 

 mer (Texas Water Development Board 1977). Because 

 about one-third of the total irrigation requirement is 

 supplied by groundwater, the volume of water released 

 is actually greater than the surface water withdrawn 

 earher in the season. Thus, surface waterflows out of 

 the basin are actually larger at present than before the 

 control structures were installed. The net effect has 

 been to modify the normal flow and water level pat- 

 terns in the basin. Figure 3-40 shows how effective 

 the control structures are in controlling water level in 

 the basin and in preventing saltwater intrusion. At the 

 northern station on Freshwater Bayou, the water level 

 inside the lock in the winter is as much as 30 cm (12 

 in) above the level one-half mile downstream outside 

 the lock (Freshwater Bayou, south). During the sum- 

 mer when Gulf water levels are higher than water 

 levels in the basin, the structures prevent intrusion of 

 salt water. 



Habitat effects. WeUand loss is occurring at an 

 annual rate of 0.88% (20,132 ha or 49,751 a from 

 1952 to 1974). All but 3,356 ha (8,293 a) can be ac- 

 counted for by direct cultural modification: impound- 

 ing of wetlands accounts for 12,797 ha (31,622 a or 

 63%) and draining for agriculture, another 2,584 ha 

 (6,385 a) 13% (table 3.69). The residual wetland loss 

 was 3.3% between 1952 and 1974, or 0.14%/yr. 

 This rate is a little higher than the residual loss rates 

 for the Vemiilion,'Chenier, and East Bay basins. Wild- 

 life biologists familiar with the basin attribute this 

 loss to erosion of lake shorehnes. This erosion results 

 from maintenance of high water levels behind the 

 control structures. Except for occasional dry years, 

 abnormally high water levels over the marshes also 

 prevent gennination of annual grasses and sedges which 

 are valuable waterfowl food (Vaughn, R. R.,U.S. Fish 

 and Wildlife Service, Atlanta. Ga., letter dated 1 April 

 1977 to District Engineer, U.S.A.C.E., New Orleans, 

 La.). 



108 



