cuts off the sediment supply. In 

 i nterdistributary basins which have very 

 little surface fresh water input, most of 

 the sediments come from tidal action. 

 Under these circumstances attempts to 

 retard salt intrusion also restrict 

 sediment input. 



In addition to marsh loss caused by 

 salt intrusion and hydrologic changes, 

 canals also directly change benthic and 

 nektonic habitat quality (Table 34). The 

 deep canals are depauperate in benthic 

 organisms because, at least in bulkheaded 

 channels, the lower part of the water 

 column and the sediments are anoxic most 

 of the year (W. Sikora, LSU Center for 

 Wetland Resources; pers. comm.). 



On the other hand, canals might enable 

 nektonic organisms to penetrate marsh 

 areas where they previously had no access, 

 although the presence of spoil banks would 

 cancel this benefit. Fish can use the 

 deep water of canals as a refuge during 

 cold spells when the shallow natural 

 streams become almost as cold as the air 

 above them. 



Habitat Qua! ity 



In the wildlife management areas of 

 the delta (Figure 71) several kinds of 

 marsh modifications are practiced to 

 improve habitat quality. Generally this 

 means improved quality for waterfowl and 

 fur animals, sometimes at the expense of 

 fishery species. But in recent years the 

 aim has been a diversified habitat that 

 will support a broad range of species. 



Where water level management is 

 active, the opening and closing of water 

 control structures is timed to increase 

 the availability of the managed area to 

 migratory fish and shellfish species. The 

 simplest control structure is the weir 

 (Figure 72); this Is a common device found 

 all over the coastal zone, especially in 

 areas managed by State or Federal 

 authorities. It is a dam placed in tidal 

 creeks to maintain a minimum water level 

 In the marshes drained by the creek. 

 Usually the top of the weir Is about 15 cm 

 below the average marsh surface. The 

 purpose of the weir is to stabilize water 

 levels to encourage the growth of 

 submerged aquatic plants and reduce marsh 



erosion by keeping the marsh from drying 

 out and oxidizing. Weirs seem fairly 

 effective for stabilizing water levels 

 (Figure 73) and for promoting growth of 

 submerged aquatic plants (Chabreck 1968). 



On the other hand, the evidence from 

 the study of Steever et al . (1976; see 

 Figure 43) that marsh plant biomass is 

 directly proportional to tide range makes 

 it likely that marsh productivity is 

 reduced by these structures. As far as 

 erosion prevention is considered, there is 

 no evidence that weirs are effective. 

 Weirs are the cheapest kind of marsh 

 management. Because of the increase in 

 submerged vegetation, the ponds behind 

 weirs attract more wintering waterfowl 

 than unwei red ponds (Spiller 1975). They 

 also improve conditions for fur animals. 



The next level of control device is 

 the flap gate and/or variable level dam in 

 a completely Impounded marsh. The flap 

 gate allows water to flow one way through 

 the control structure. Modern ones are 

 reversible, but in Louisiana, with its high 

 rainfall, they are usually set to allow 

 freshwater to flow out of the impoundment 

 and to prevent saltwater from moving in. 

 Because of the surplus rainfall, all 

 impounded areas become fresher with time. 



The variable height device, which is 

 often incorporated in the same structure, 

 allows the manager to set minimum water 

 levels behind the weir. With this 

 "gravity drainage" systen, if the weather 

 cooperates it is possible to draw down the 

 water In the spring to allow seeds of 

 annual emergents to germinate. It can 

 then be raised in the winter to make 

 shallow ponds for ducks. 



The most sophisticated water level 

 control Is obtained by pumping water out 

 of or into the impoundment (forced drain- 

 age). The effectiveness of these manage- 

 ment measures can be judged by the kinds 

 and diversity of vegetation produced 

 (habitat quality) and the use of the 

 impoundment by birds, fur animals, fish, 

 and shellfish. 



Wicker et al . (1983) summarized the 

 effectiveness of impoundments in the 

 Rockefeller Wildlife Refuge. Annual 

 vegetation surveys carried out since 1958 



93 



