followed by 1 to 2 years of no
so that natural competitive
interactions between drawdown-sensitive and insensitive species remain and the
Winter drawdowns are the most success-
less sensitive do not become dominant.
ful for northern reservoirs, interfere
with other reservoir uses, and
In general, the long-term effects
should refill promptly with spring runoff.
of
on aquatic plants are poorly understood.
The feasibility of this method for a particular reservoir is dictated
Long-term
could not be used in a hydropower
largely by its use.
Many of these reservoirs, however, have few
reservoir, for example.
phyte problems due to the absence of shallow areas and to the large water
The level of mainstream reservoirs is
level fluctuations over short periods.
dominated by riverflow and the amount of water level manipulation possible is
sometimes limited (Ploskey, Aggus, and Nestler 1984).
High-frequency,
Bates, and Stanley (197-8) at Melton
duration withdrawals, as used by
Flood control impoundments
Hill, could be used for this type of reservoir.
are good candidates for water level drawdowns, particularly during winter
months.
Comparatively very low costs are associated with this procedure, and it
is possible that the implementation of other techniques during the
could produce some cost savings in overall reservoir management.
Limitations and Concerns
Ploskey (1983) lists several ways in which
can interfere with
reservoir uses, including interference with navigation, access for boaters and
Most, if not all, of the problems are
swimmers, fishing, and fish management.
averted by winter drawdowns and spring refills.
Algal blooms after reflooding
were reported by Hulsey (1958) and Beard
although the causes of such
blooms are poorly understood. High external loading, release of nutrients
from sediments, and elimination of competitive effects of higher plants with
Spring and summer drawdowns can have several neg-
algae may all be involved.
These include destruction of lit-
ative effects on fishing (Ploskey 1983).
toral food organisms, elimination of cover, and interference with spawning.
dissolved oxygen in the remaining pool can produce a fish kill, or
Also,
summer drawdowns can eliminate thermal stratification and introduce anoxic
waters to the entire reservoir (Geagan 1960, Richardson 1975, Shaw 1983).
One
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