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2.9 Reservoir Management 47
carted away; (c) trees and brush are cut close to the ground, usable timber is sal-
vaged, and slash, weeds, and grass are burned; (d) swamp muck is dug out to rea-
sonable depths, and residual muck is covered with clean gravel, the gravel, in turn,
being covered with clean sand; and (e) channels are cut to pockets that would not
drain when the water level of the reservoir is lowered.
2. Within a marginal strip between the high-water mark reached by waves and a con-
tour line about 20 ft (6.1 m) below reservoir level: (a) Stumps, roots, and topsoil
are removed; (b) marginal swamps are drained or filled; and (c) banks are steep-
ened to produce depths near the shore that are close to 8 ft (2.44 m) during much of
the growing season of aquatic plants—to do this, upper reservoir reaches may have
to be improved by excavation or fill or by building auxiliary dams across shallow
arms of the impoundage.

Soil stripping, namely, the removal of all topsoil containing more than 1% or 2% or-
ganic matter from the entire reservoir area, is no longer economical.
In malarious regions, impounding reservoirs should be so constructed and managed that
they will not breed dangerous numbers of anopheline mosquitoes. To this purpose, banks
should be clean and reasonably steep. To keep them so, they may have to be protected by riprap.

2.9 RESERVOIR MANAGEMENT

The introduction of impounding reservoirs into a river system or the existence of natu-
ral lakes and ponds within it raise questions of quality control. Limnological factors are
important not only in the management of ponds, lakes, and reservoirs but also in reser-
voir design.

2.9.1 Quality Control
Of concern in the quality management of reservoirs is the control of water weeds and algal
blooms; the bleaching of color; the settling of turbidity; destratification by mixing or aera-
tion; and, in the absence of destratification, the selection of water of optimal quality and
temperature by shifting intake depths in order to suit withdrawals to water uses or to down-
stream quality requirements.

2.9.2 Evaporation Control
The thought that oil spread on water will suppress evaporation is not new. It is well
known that:
1. Certain chemicals spread spontaneously on water as layers no more than a mole-
cule thick.
2. These substances include alcohol (hydroxyl) or fatty acid (carboxyl) groups
attached to a saturated paraffin chain of carbon atoms.
3. The resulting monolayers consist of molecules oriented in the same direction and
thereby offering more resistance to the passage of water molecules than do thick
layers of oil composed of multilayers of haphazardly oriented molecules.
4. The hydrophilic radicals (OH or COOH) at one end of the paraffin chain move
down into the water phase, while the hydrophobic paraffin chains themselves
stretch up into the gaseous phase. Examples of suitable chemicals are alcohols and
corresponding fatty acids.