Flushing, in contrast to dilution, does not require that the nutrient
concentration in the inflowing water be less than that of the reservoir.
Quantity of algal cells is controlled not by nutrient limitation but by wash-
The flushing rate therefore must be close to the algal growth rate to be
out.
effective.
Flushing rates of 10 to 15 percent per day are believed to be suf-
ficient (Cooke et al. 1986).
Effectiveness, Costs, and Feasibility
There are only a few published case histories of the use of dilution/
state, and only two have substantial long-term
flushing to improve
documentation.
These two are Moses Lake, Washington (Welch, Buckley, and Bush
1972; Welch 1979, 1981; Welch and Patmont 1979, 1980; Welch and Tomasek 1980;
Welch, Brenner, and
1984; Cooke et al. 1986) and Green Lake, Washing-
1968,
Welch 1981; Cooke
ton (Sylvester and Anderson 1964;
et al. 1986).
The feasibility of this method for reservoir improvement is very limited
since an adequate supply of low-nutrient dilution water or high flows of addi-
tional water for flushing are unlikely to be available in most instances.
Further, even if there is a potential supply of water, its use for reservoir
dilution/flushing may be restricted by prior usage of the water.
Since this
method of reservoir improvement is likely to have limited use, the results of
the Moses and Green Lakes studies will only be briefly reviewed.
Moses Lake, Washington
Crab Creek, the primary water supply to this large
rela-
tively shallow (mean depth, 5.6 m) lake in eastern Washington, has very high
nutrient Content.
In 1977, dilution water addition to Parker Horn began,
using low-nutrient Columbia
that was diverted through Moses Lake
and thence to agricultural areas for irrigation. This produced overall water
-1
-1
for Parker Horn, and 0.01 to 0.02 day
exchange rates of 0.1 to 0.16 day
In 1982, dilution water was pumped to previously
for the whole lake.
undiluted Pelican Horn from Parker Horn.
The percent lake water in Parker Horn dropped to less than 30 percent
-1
when the dilution rate reached 0.15 day
Dramatic improvements in lake
However, it
quality occurred, not only in Parker Horn but in the entire lake.
was obvious that algal blooms and low transparency returned quickly if the
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