18 BULLETIX 109S, U. S. DEPARTIMEXT OF AGRICTTLTUEE. 



of the vegetable shelter, which operates against the concentration of 



adults along the bayou and consequent oviposition: and depletion 

 of the larval food of Anopheles furnished by the decaying vegetation 

 and the Iotv forms of aquatic life, both plant and animal, common to 



the swamplike conditions of the natural bayou. 



LEAKAGES CAUSED BY CRAWFISH AND MEANS OF PREVENTING 



THEM. 



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

 about the boxing of the spillway 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. The action of the water through these 

 openings in carrying away the dirt caused serious leakage, which re- 

 sulted in a decidedly lower level of 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 fol- 

 lowing years, up to 1920, considerable expense was involved in pre- 

 venting 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 from working to the outside, below the fill. 

 and to hold the water above the dam at a permanent level. 



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 re- 

 maining 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 semiaquatic vegetation 

 that had gained a foothold. Then when the leakage had been re- 

 paired and the water level raised to its original height, it rested 

 against comparatively clean margins. The growth of marginal vege- 

 tation was thus discouraged by this variable water level, and this ex- 

 plains the lack of any maintenance work on control of marginal 

 vegetation in the impounded area. Thus, the expense in the mainte- 

 nance work on the dam was ofi'set in part by the saving in the work 

 on the margins. 



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 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 is provision for controlling the water 

 level above the dam. The object of this is to make use of the effect 



