Water storage by the marsh helps reduce fluctuation in water
flow.
Adverse effects:
of high spring or fall
Nutrients are released during
flows, necessitating nutrient interception.
Nitrogen fixation occurs at a high rate, producing an increase in
l
concentration of organic nitrogen in the marsh discharge.
Water leaving a marsh may
and odor problems.
l
Marsh discharges may be high in organic matter and color, and low
l
in dissolved oxygen.
Diversionary streams
Storm events often exhibit the poorest water quality and can represent a
very significant fraction of'the total annual loading of nutrients, silt, and
In some situations, it may be possible to
organic matter to the reservoir.
divert some or all of these high flows around the reservoir, especially in
reservoirs used solely for recreation or as water supplies, and not for flood
The use of this procedure has not been docu-
control or power generation.
mented with regard to determinations of when or how to divert the water, the
effectiveness of the procedure, nor the impact on downstream biotic com-
munities.
These data will vary from case to case, and a detailed budget of
silt and nutrient loading will form the basis of any design to divert high
flows.
Effectiveness, Costs, and Feasibility
Siltation basins
The work of Fiala and Vasata (1982) provides an example of the effec-
tiveness of a siltation basin in removing phosphorus from incoming waters.
Jesenice Reservoir, Czechoslovakia, was divided into a small (area = 76 ha;
volume = 1.4
siltation basin with a detention time of 5 days. It
emptied into the main reservoir (area
670 ha; volume
51
which
had a theoretical hydraulic detention time of 180 days. Orthophosphorus fell
-1
-1
from over 500
P
at the inlet of the siltation basin to 30
P
at
-1
its outlet. Orthophosphorus then reached about 10
P
at the pool behind
the main dam.
Phytoplankton biomass also declined.
The authors note that
phosphorus retention by the siltation basin increased with detention time, and
they suggest a minimum of 5 days.
This could be difficult to achieve on an
36
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