Hydroelectricity - Energy Policy Analysis: A Conceptual Framework - page 150

This can result in further changes to a riparian zone and the species that it can support. Increases
in erosion can also increase the amount of sedimentation behind a dam (FWEE 2011).
Siltation. Flowing water transports particles heavier than itself downstream. Sediments, which
are fine organic and inorganic materials that are typically suspended in the water, can collect behind
a dam. Human-made and natural erosion of lands adjacent to a reservoir can lead to sediment
buildup behind a dam. This buildup can vary based on the ability of a river to flush sediments
past a dam (FWEE 2011).
When sediments collect, an ecosystem can be affected in two ways. First, downstream habitat
can decline because these sediments no longer provide important organic and inorganic nutrients.
Second, where sediment builds up behind a dam, the supply of oxygen may be depleted. This
happens because more nutrients are available, and more organisms populate the area to consume
the nutrients. As these organisms consume nutrients, more oxygen is used, depleting the supply
of oxygen in the reservoir. Gravel can be trapped behind a dam in the same way as sediment. In
cases where the movement of gravel downstream is part of establishing spawning areas for fish,
important habitat conditions can be affected (FWEE 2011).
This has a negative effect on dams and subsequently their power stations, particularly those on
rivers or within catchment areas with high siltation, which can fill a reservoir, reduce its capacity
to control floods, and cause additional horizontal pressure on upstream portions of a dam (Chanson
and James 1998; Sentürk 1994, 375). Eventually, some reservoirs can become completely full of
sediment and useless, or they allow water to flow over the top of accumulated sediment during
a flood and fail.
Low Flows. Changes in the amount of river flow correlate with the amount of energy produced
by a dam. Lower river flows because of drought, climate change, or upstream dams and diversions
will reduce the amount of storage in a reservoir, thereby reducing the amount of water that can be
used for hydroelectric generation. The result of diminished river flow can be power shortages in
areas that depend heavily on hydroelectric power. The risk of low flows may increase as a result
of climate change (Urban and Mitchell 2011).
Wildlife. Riparian vegetation and its bordering waters provide critical habitat for birds, wa-
terfowl, and small and large mammals. When a hydroelectric project results in inundation of a
free-flowing river, the nesting, forage, and cover provided by these areas is temporarily or per-
manently lost.
When habitat is lost, animals are forced to move to higher ground or other areas where habitat
conditions may be less suitable, predators are more abundant, or the territory is already occupied.
As an example, ground birds like pheasant and grouse require cover and cannot successfully move
to higher, more open, ground. In cases where water levels stabilize at a new height, vegetation
in riparian zones can reemerge and species can repopulate an area. With storage projects, the
riparian zone that reemerges has conditions that reflect those of a reservoir or lake rather than a
free-flowing river. When such conditions occur, certain species will decline, others will become
more abundant, and some will populate these areas for the first time.
Ducks and geese are examples of waterfowl that are strongly attracted to habitat conditions
found in reservoirs. For some of these species, reservoirs are providing an important alternative to
the wetland areas that they formerly occupied. Canada geese, for example, now frequent reservoirs
as part of their migration pattern (FWEE 2011).
Hydroelectric power plants affect various fish populations in different ways. Dams along the