MANAGEMENT OF WATER FOR MALARIA CONTROL. 
327 
two distinct lines. A slow, continuous 
drawdown throughout the entire breeding 
season may be efficacious if of sufficient 
scope. Such a procedure is known as sea¬ 
sonal recession. On storage reservoirs 
where a wide drawdown fits into the re¬ 
quirements for power production, naviga¬ 
tion or flood control, this simple measure is 
effective. However, there are many proj¬ 
ects on which only a limited scope of water 
level variation is permissible. If stream 
flow or upstream reservoirs permit a regu¬ 
lar pool level variation between constant 
limits, a cyclical fluctuation may be pro¬ 
vided. This consists of a drawdown of a 
definite scope and refilling to previous ele¬ 
vation within a regular interval. Thus the 
elevation of the lake might be lowered one 
foot over a period of 3.5 days and refilled 
a foot over the same period. This would 
constitute a weekly cyclical fluctuation in 
level of one foot. The interval might be of 
any length but probably should not exceed 
10 days (it should be less than the length 
of life of the larval stage of the mosquito). 
Such a type of fluctuation is particularly 
applicable where reservoirs are located in 
tandem and the drawing down process in 
one lake may coincide with the filling of the 
one immediately below it. However, any 
project utilized as a hydroelectric develop¬ 
ment will inevitably undergo a certain vari¬ 
ation in water level. If this can be sched¬ 
uled to be periodic it will be beneficial in 
preventing the development of anopheline 
larvae. 
On the Tennessee Valley Authority lakes 
a combination of seasonal recession and 
cyclical fluctuation has been found to be 
most applicable under the conditions of 
operation. The graph at the end of this 
paper illustrates this type of schedule. The 
reservoir is lowered a definite amount dur¬ 
ing the first half of the cycle but the re¬ 
filling process does not take it back to the 
original level, e.g., lowered 1.0 but refilled 
only 0.9. This results in a “ staircase ’ ’ type 
of schedule which has been found quite 
effective with a limited periodic fluctuation. 
The mechanics by which fluctuation of 
pool level exerts its beneficial influence is 
not known exactly, but certain desirable 
effects are obvious. Each individual weekly 
drawdown, whether it be part of the cyclical 
fluctuation or of the seasonal recession, 
accomplishes several things, including 
strandage of debris which may have entered 
the reservoir recently, and strandage of 
larvae, pupae or eggs at the margin. In 
addition, certain larvae may be drawn out 
into open water where minnows and other 
predators have a greater opportunity to 
devour them. Finally, the drawdown 
diminishes the total area of potential breed¬ 
ing surface. 
Cyclical fluctuation has a profound effect 
upon vegetation within the fluctuation zone. 
The drawdown, by its temporary dewater¬ 
ing, makes this zone uninhabitable by cer¬ 
tain species of submerged vegetation. Re¬ 
peated refillings tend to exert a retarding 
influence on those species of terrestrial 
vegetation which would tend to advance 
constantly with the drawdown. Cyclical 
fluctuation also exerts an influence upon 
various species of erect emergent plants 
such as Saururus cernuus (lizard’s tail) 
and Dianthera americana (water willow 
weed). It appears to defoliate the lower 
portions of stems in the water, thus pro¬ 
ducing less protection for the larvae and 
making the condition ' less attractive to 
them. Conversely, when reservoirs are 
filled above the scheduled elevations, the 
leafy portions of such vegetation are inun¬ 
dated and a higher density of larvae may be 
observed. 
Unfortunately fluctuation of water level 
is not effective against all species of plants. 
Those types which are equally well adapted 
to either the terrestrial or the aquatic en¬ 
vironment thrive when there are variations 
of water level. Similarly, those species 
which are suited to growth in deep water, 
e.g., Nelunibo lutea (lotus) and Nympheae 
advena (cowlily), are not inhibited. 
Regardless of the exact mechanism by 
which fluctuation of water level exerts its 
beneficial effect, it should be considered 
as prophylactic rather than palliative. 
Through the control of water level the ob¬ 
jective becomes one of making the environ- 
