76 Procekdings of Nin-th Annual Mee:^ing 



the natural bayou and from all other classes of surface water, 

 we must consider the effect of great dilution. It is sufficient for 

 the practical results of the work to note that Gambusia survived 

 the effect of impounding in important numbers, and that the 

 presence of the game fish in the area serves the purpose of keeping 

 them along the margins where they are useful in the marginal 

 control of mosquito breeding. 



The non-breeding of the Ampheles in the impounded water ig 

 due to a number of factors which have not as yet been definitely 

 measured. In general, as has been stated, the important differ- 

 ence between the impounded section of the bayou and the natural 

 bayou is just the difference between lake-like conditions which 

 do not favor the development of Anopheles and swamp-like con- 

 ditions which do favor such development. The lake-like condi- 

 tions of the impounded section are shown in figures 7 and 8. 

 The factors which are considered to operate against mosquito 

 development in the impounded water are the greater freedom 

 for action on the part of the predators, the fish and the aquatic 

 insects; wave action; depth of water which influences the tem- 

 perature ; absence of the vegetable shelter along the bayou which 

 operates against the concentration of adults and consequent 

 oviposition; and the depletion of the larval food of Anopheles 

 furnished by the decaying vegetation and the low forms of 

 aquatic life, both plant and animal, common to the swamp-like 

 conditions of the natural bayou. 



The work of maintenance at the dam due to the action of the 

 crawfish has been mentioned. The crawfish burrowed through 

 the fill below the level of the water above the 'dam to the lower 

 side of the fill. This injury occurred about the boxing of the 

 spillway. The action of the water through these openings in 

 carrying away the dirt of the fill caused serious leakage which 

 resulted in a decided lower level of the water above the dam. In 

 several instances the level of the bayou was lowered materially 

 before proper repairs in the dam were made. This damage was 

 not serious the first year following the completion of the dam 

 but during the following years, up to 1920, considerable expense 

 was involved in preventing the leakage in the dam due to the 

 work of the crawfish. In 1920 a double course of sheet piling 

 with overlapping joints was driven below the fill, leaving an 

 opening for the spillway, the boxing of which was carried 

 through and over the sheet piling. This served to prevent the 

 crawfish working to the outside, below the fill, and to hold the 

 water above the dam at a permanent level. 



