very significant problem is the failure to refill due to an unexpected drought
or to poor timing of the
relative to expected rainfall.
Winter
drawdowns for flood control projects have a low
refill problems
due to the usually high volume of spring runoff.
Summer drawdowns can remain
if autumn is dry.
An absence of refill
minimal negative impact
on reservoir users and the fishery, and the best plant control are most likely
to be achieved with the use of winter drawdowns, particularly in cold
climates.
Summary
Water level
is an effective procedure for the control of cer-
tain species of nuisance aquatic macrophytes.
Control is achieved through
drying and freezing over a period of at least 3 to 4 weeks for projects
located in northern areas.
Somewhat longer periods may be required for
southern projects.
Plants that remain in moist soil or in shallow water can
be expected to survive.
can also be used to implement other procedures, including
repair or installation of structures for control of shoreline erosion or for
Also, exposed, consolidated sedi-
gaining access to dams, docks, and piers.
ments can be more easily removed with earth-moving equipment, assuming suffi-
cient consolidation to bear weight, than by removal during normal water level
Water level manipulation can also be used for fish
with a hydraulic dredge.
management and to facilitate the installation of sediment covers.
Negative aspects
method primarily involve problems of access to
water by reservoir users, failure to refill, and possible effects on fish-
Most of these are avoided at many projects by use of winter drawdowns.
eries.
This procedure cannot be used at all reservoirs since only some types of oper-
is sum-
Water level
ations will permit long-term winter drawdowns.
marized in Table 11.
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