Environmental Engineering Reference
In-Depth Information
in the epilmnion (Spellman, 1996, 2008). In temperate regions, the decline in hypo-
limnetic dissolved oxygen concentrations begins at the onset of stratification (spring
or summer) and continues until either anaerobic conditions predominate or reoxy-
genation occurs during the fall turnover of the water body.
Numerous structural, operational, and regulatory techniques are available to
resolve low dissolved oxygen issues. Levels of dissolved oxygen can be increased
through modifications in dam operations that include such techniques as fluctuating
the timing and duration of flow releases, spilling or sluicing water, increasing mini-
mum flows, flow mixing, and turbine aeration; at some sites, injection of air or oxy-
gen for weir aeration has proven effective. The most effective strategy for addressing
the dissolved oxygen problem depends on the site.
ECOLOGICAL IMPACTS OF HYDROPOWER
During operation, adverse ecological effects could result from disturbance of wild-
life by equipment noise, site inspection and maintenance activities, exposure of biota
to contaminants, and mortality of birds due to collisions with the project facilities
and/or electrocution by transmission lines. During operation, wildlife could still
be affected by habitat fragmentation or the presence of barriers in fenced areas,
canals, or above-ground pipelines, utility rights-of-way (ROWs), and access roads.
In addition, the presence of the hydropower project and its associated access roads
and ROWs may increase the human use of surrounding areas, which could, in turn,
impact ecological resources in the surrounding areas through
•
Introduction and spread of invasive vegetation
•
Disturbance of habitats
•
Mortality of wildlife from vehicles
•
Increase in hunting (including poaching)
•
Increased potential for fire
•
Physical barriers to fish migration
•
Flow alteration and fluctuations
•
Biological impacts of flow fluctuations
In the discussion that follows, for the purposes of this text, we concentrate on the
last three impacts listed above related to the Pacific Northwest region of the United
States.
p
HysICaL
b
arrIer
to
f
IsH
m
IgratIon
The presence of a dam could impose a physical barrier to fish migration. As described
earlier in the Rachel River account, this could be mitigated by construction of a fish
ladder (see
Fig u re 4.11
).
Figure 4.12
s
hows a window view of salmon climbing a fish
ladder to their spawning grounds in the Pacific Northwest. In this area, salmon runs
include sockeye salmon in June (see
Figure 4.13
)
, Chinook salmon (see
Figure 4.14
)
and coho salmon (see
Figure 4.15
) in September and October, and steelhead (see
Figure 4.16
)
in late fall and winter in Lake Washington, Seattle, Washington.
