PART III:
PRERESERVOIR TREATMENT
Problem Addressed
Dense populations of algae, particularly blue-green algae, can create
nuisance conditions in reservoirs, which can have negative impacts on project
uses.
algal "blooms" are caused by a combination
favorable condi-
warm water, and high nutrient concentrations. A
tions of light, high
second reservoir problem involves the loss of basin volume through the depo-
This process creates shal-
sition of silt and organic matter from the land.
low, well-lighted, nutrient-rich areas for macrophyte growth,
It also leads
Both of these problems are related to the transport
to impaired project use.
of material from watershed to reservoir.
Nutrient concentration in the water, as described in an earlier part and
is a function of nutrient income, loss to sediments and
in Walker
outflow, dilution by basin volume, and release from sources inside the reser-
When nutrient income is reduced through advanced waste treatment or
voir.
land management of urban and agricultural flows, concentration in the reser-
blooms may decrease or be eliminated.
The diver-
voir may decline and
sion of nutrients from Lake Washington (Edmondson
is an example of this
Similarly, land management to control erosion can curtail
type of response.
silt income and reduce the rate of basin volume
However, nutrient diversion or advanced treatment and land management
are often impossible to effectively accomplish in reservoir management.
The
drainage basin is usually very large, cutting across many political bounda-
ries.
This makes action to create lowered nutrient and sediment income very
difficult or impossible for the reservoir manager. An alternative or an addi-
tion to advanced waste treatment and land management is to pretreat the water
from incoming streams through the construction of structures to accelerate
nutrient and silt sedimentation, or to add substances to the incoming stream
water to precipitate nutrients and particulate matter.
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