MANAGEMENT OP WATER FOR MALARIA CONTROL 
325 
levels; and (2) that all depressions between 
maximum and minimum water levels be 
drained so as to provide water level fluctu¬ 
ation with the lake. ’ ’ 
In preparing for most reservoirs it is 
necessary that the entire area to be flooded 
should be cleared of all trees and that all 
timber be removed. This is not only a 
necessary anti-larval measure, but it may 
be a requisite for removal of hazards to 
navigation and sources of drift which may 
be objectionable at the dam. In addition to 
the general clearing which extends up to 
the normal maximum summertime water 
elevation, there are certain special types of 
clearing, including (1) erosion clearing— 
carried back from 6 to 20 feet horizontally 
along steep shoreline to prevent trees fall¬ 
ing into the lake; (2) clearing at heads of 
bights and indentations to provide space 
for stranding and removal of drift and 
flotage; (3) clearing of islands where they 
would be exposed to wave action and subse¬ 
quently erode; (4) clearing for airplane 
dusting on points and at heads of indenta¬ 
tions to facilitate larvicidal application; 
(5) clearing in the flood surcharge zone. 
Experience has shown that certain species 
of trees are intolerant to inundation and 
that temporary flooding during the grow¬ 
ing season results in their death. Since 
these dead trees may fall into the reservoir 
and create a mosquito breeding hazard it is 
advisable to remove them prior to impound- 
age. Botanical studies carried on in the 
Health and Safety Department of the Ten¬ 
nessee Valley Authority have resulted in a 
classification of all woody species with ref¬ 
erence to their ability to live under ex¬ 
tended periods of inundation as: tolerant; 
semi-tolerant; and intolerant. In clearing 
practices only the latter group are removed 
from the flood surcharge zone. (6) Adjust¬ 
ment of clearing line to anticipated back 
water curves. In long narrow reservoirs 
having a considerable volume of inflow 
there may be a difference of several feet in 
water level between the lower and upper 
parts of the lake. In such instances it may 
be desirable to clear to higher elevations in 
the upstream portion of the reservoir. (7) 
Ten-foot zone conditioning above basic 
clearing line to remove dead and diseased 
trees, leaning trees, brush, logs, limbs, and 
debris lying on the ground to preclude the 
entrance of such material into the reservoir 
following impoundage. 
The disposal of timber may be through 
sale, by piling and burning or, if below the 
minimum lake elevation, it may be fastened 
to the ground. The piling of timber along 
the margin should be discouraged since it 
creates a fire hazard. In marginal areas it 
is necessary that precautions be taken dur¬ 
ing the burning operation not to damage 
the standing timber by fire, since such trees 
may become diseased, die and eventually 
fall into the reservoir. 
In large reservoirs the clearing opera¬ 
tions may extend over one or several grow¬ 
ing seasons, during which time most of the 
stumps will produce new shoots. Such 
coppice will constitute a favorable environ¬ 
ment for Anopheles breeding following the 
filling of a reservoir. However, it is only in 
the marginal areas, where it reaches a 
height sufficient to break through the water 
surface, that it becomes a problem. In 
these areas also a rank growth of weeds is 
often produced. To combat this the Ten¬ 
nessee Valley Authority, since 1937, has 
pursued the policy of reconditioning the 
marginal zone during the autumn prior to 
filling, which should be done during the 
winter or early spring. This includes the 
cutting of all sprouts and larger herbaceous 
vegetation. Through the practice of low 
cutting of stumps it has been possible to 
use agricultural implements for this work. 
Either horse or tractor-drawn mowers and 
rakes make the operation a very economical 
one. After the material is dried it burns 
readily in windrows. By such treatment a 
clean shoreline results with marked savings 
in larvic-ides during the initial season. 
One species of tree has shown amazing 
ability to sprout from stumps, namely the 
black willow (Salix nigra). Even though 
the stumps be submerged for the major 
portion of the breeding season, as soon as 
the water level is drawn down sufficiently 
to expose them young shoots appear. If 
