18 BULLETIN 1098, U. S. DEPARTMENT OF AGRICULTURE. 
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 low 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. Im 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 débris 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 offset 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 
