N. J. Mosquito Extekmination Association 77 



An important biological observation was made in connection 

 with the variable water level caused by the leakage in the dam due 

 to the crawfish. It was found that when the water was lowered, 

 after remaining at one level for a period, the water found a 

 clean edge free from debris and grass and, further, that the 

 drying out above the new level served to destroy the aquatic and 

 semi-aquatic vegetation that had gained a foothold. Then when 

 the leakage had been repaired and the water level raised to its 

 original height, it rested against comparatively clean margins. 

 The growth of marginal vegetation was thus discouraged by 

 this variable water level which explains the lack of any mainte- 

 nance work on marginal control in the impounded area. Thus 

 was the expense in the maintenance work on the dam offset in 

 part by the saving in the work on the margin. 



The experience with the crawfish suggests two improvements 

 to be considered in any further work on impounding water in a 

 bayou in this region. The first point is the prevention of injury 

 to the fill in the dam on the part of crawfish. This can be 

 accomplished by a core-wall extending below and to each side of 

 the spillway box in the center of the fill. The second point is 

 provision for controlling the water level above the dam. The 

 object of this is to make use of the effect of a variable water 

 level on the rharginal vegetation and marginal mosquito breed- 

 ing. This is obtained by a change 'in the height of the water 

 from time to time which can be accomplished by a sluice-way 

 through the fill below the level of the floor of the spillway. The 

 flow of water through the sluice-way can be controlled by a gate. 

 If the sluice-way is placed at the level of the bed of the bayou, 

 in the center of the fill, it would act efficiently in lowering or rais- 

 ing the water level above the dam, and, also, the current of water 

 through the sluice-way at this point, would flush out and away 

 the mud and sediment that tends to accumulate in the bed of 

 the bayou immediately back of the fill. 



Advantages of the plantation from the impounding work, 

 apart from the control of Anopheles breeding in a nearby source 

 of these mosquitoes, are a permanent supply of good water for 

 live-stock during the dry season; and extension of the land 

 available for pasture and the pasturing, in turn, aiding in the 

 suppression of vegetation along the stream ; the deeper and more 

 extensive water of the impounded area offers a favorable place 

 for game fish and thus furnishes a source of food for the ten- 

 ants; and the lake-like body of water offers recreation to the 

 plantation people and adds to the attractiveness and the value of 

 the property. 



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