Changes in the Physical-Chemical
Environment
Upstream Effects
The major upstream effect of the dam is the creation of a reservoir or
a controlled stretch of river. In general, a considerable body of water
is stored in the reservoir behind the dam. When water is stored for some
time in large reservoirs it may become stratified, whereby the warmer
waters (that are less dense) float on the cooler (more dense) waters in
the deeper parts of the reservoir. The surface waters are further warmed
by the sun and, as the density of water decreases with rising temperatures,
mixing of the cooler mass of water at the bottom is prevented. As a result,
stratification of the water column may occur.
In these circumstances, there is a strong seasonality whereby the water
mass at the bottom may become deoxygenated and unable to support life,
although the surface waters may be well oxygenated. The lake may or may
not return to normal when storms in the monsoon cause the water at the
bottom to mix with that at the surface, and produce a more uniform concentration
of oxygen. Deep reservoirs may become permanently stratified and de-oxygenated.
New reservoirs show an increased tendency to become anoxic in the period
immediately after filling when the large quantities of vegetation flooded
by the rising water start to decay. This phase may last for several months
or even years, particularly where the original area covered by the reservoir
was forested. The tendency for reservoirs to become deoxygenated is also
accelerated in regions where there is a substantial input of nutrients
from agricultural fertilizers, animal wastes or human sewage, either directly
into the reservoir or into the river feeding it.
The warm upper waters of a reservoir are well lit by the sun's rays and,
in contrast to the swiftly flowing river waters, can support abundant
phytoplankton (see Section 6) that produce oxygen by photosynthesis. In
contrast, the deeper waters receive little light, and phytoplankton are
scarce. Organic matter – mainly FPOM – carried by the river is deposited
on the bottom of the reservoir and provides an energy source for microbes – especially bacteria (see Section 6). They consume oxygen during the
process of decomposition and, since the water column does not mix, the
oxygen in the deeper water is gradually depleted leading, in extreme cases,
to deoxygenation. When this happens, most aquatic life becomes confined
to the surface waters. Deoxygenation of reservoir waters is particularly
common soon after a dam has been constructed, since the filling phase
of the reservoir tends to involve inundation of large amounts of terrestrial
organic matter. This fuels the development of large populations of microbes
that quickly deplete deeper waters of oxygen.
During flood periods, the still waters of the reservoir may be mixed by
flows of water from upstream; strong wind blowing across the water surface
can have the same effect.
Regardless, such periodic stratification and deoxygenation of deep waters
that may occur during much of the year is very different from the continual
mixing of surface and bottom waters that occurs in even the deepest rivers.
The extent and duration of reservoir stratification is variable and depends
on the shape of the reservoir; those constructed in deep valleys are more
likely to become stratified and deoxygenated than those that are shallow
or have a large surface area relative to their volume.
Downstream effects of dams on rivers can occur immediately downstream
of the dam, or may persist for considerable distances. In extreme cases,
the dam may hold all water and prevent any flow to escape downstream;
this causes dewatering and all aquatic life perishes in the drawdown zone.
More commonly, at least a proportion of the natural flow volume is allowed
to pass the dam. The condition of this water depends upon whether it has
been released from the surface of the reservoir, or if it comes from the
reservoir depths. Surface waters are usually clear, warm and oxygen-rich,
and contain little detritus (in the form of FPOM) but may have much more
phytoplankton than would be found in river waters. Water released from
the depths of the reservoir, on the other hand, is cool and oxygen poor,
and contains little or no phytoplankton. Accordingly, animals downstream
of the dam may have to deal with elevated temperatures but enjoy a rich
food supply (in the form of phytoplankton), or may have to cope with cool
conditions, low oxygen and little food.
The distance that the effects of a dam are felt downstream depend on the
extent to which the temperature of the reservoir waters vary from (above
or below) ambient river temperatures, as well as the distance that it
takes for filter feeders (such as hydropsychids and simuliids; see Section
6) to consume the phytoplankton. However, there is an additional effect
of dams. Regardless of whether water is released from the surface or depths
of the reservoir, the water will contain little suspended sediment. Once
the water starts to move downhill in the channel below the dam, it has
a tendency to erode and transport material from the river bed or banks
creating an erosion zone that often extends for several kilometres below
the dam. This means that fine sediments, as well as sand and gravel, may
be eroded, and these substrates, which may be important to spawning fishes,
are often scarce in river sections downstream of dams.
One of the most fundamental biological effects of a dam across a river
is the disconnection of migratory pathways by which fish and other animals
such as shrimps travel up- and downstream. This may result in the elimination
of certain species that make upstream migrations for breeding above the
dam, or may cause a drastic decline in their abundance. Dams may also
lead to the fragmentation of populations, leading to genetic segregation
that may deplete biodiversity (see Section 8). Note that dams do not merely
block migrating adult fishes. Species that spawn upstream may rely on
the current to transport their eggs and larvae downstream. The slower
water in reservoirs means that these fish eggs and larvae that are normally
carried by the current in rivers settle to the bottom and may die. Therefore
reservoirs and dams pose a barrier to downstream migration of young fish,
as well as to the upstream migration of the adults.
Downstream Effects
Downstream effects in rivers can include impacts immediately below the
dam, or may persist for considerable distances along the course of the
system.
Short Distance Effects. Water discharges into the river downstream
of the dam through three sources:
The water used for operating the powerplant in the case of hydroelectric reservoirs, or releases to empty the reservoir after a flood event in the case of flood control reservoirs. (In reservoirs for urban supply and agriculture the water may never return to the river or may do so at a considerable distance downstream.);
Water used to operate any fish-passes associated with the dam; and
Excess water released over the spillway.
The positioning of the spillway is very important in determining the effects of dams on the tailwaters immediately downstream. Spillways situated at the top of the dam discharge highly oxygenated, warm water into the river downstream. Spillways situated near the bottom of the dam release cold, deoxygenated water.
Physical,
chemical and biological changes occur in the water stored in reservoirs.
As a result the water discharged at a dam can be of different chemical
composition and show a different seasonal pattern to that of the natural
river. Generally, the reservoir acts as a nutrient sink, storing much
of the inflowing nitrogen and phosphorus in the sediment. Some chemicals
may increase, for example, in arid regions the water may become saltier
through evaporation, particularly in areas where the inflowing streams
have a high salt content.
Reservoirs also influence the temperature of the river immediately below
the dam and, although this alters the ambient temperature as the water
moves downstream, the thermal impact of the dam can be detected for a
considerable distance downstream. Water temperature is important for the
assessment of impacts of a dam and reservoir on downstream aquatic habitats,
because it influences many important physical, chemical and biological
processes. As a result, the thermal change may have significant effects
on the living organisms in the river.
The quality of water discharged from a stratified reservoir depends on
the position of the outflow relative to the different water layers in
the reservoir. When the water is stratified, water released from near
the surface will be well oxygenated, warm and depleted in nutrients. Water
released from near the bottom will be cold, oxygen-depleted and nutrient-rich.
It may also contain toxic contaminants such as hydrogen sulphide, iron
and manganese.
In both upper and lower intakes, the water is likely to have a low silt
loading, although water discharged from near the bottom is more likely
to contain some silt. The low silt loading relative to flow means that
the water has a tendency to pick up more suspended matter from the bottom
of the river, leading to an erosion zone that often extends for a considerable
distance below the dam. The bottom of the river is cut down rapidly in
this erosion zone, and often bottom substrates such as gravel, important
for the spawning of fish, are carried away.
Long Distance Effects. The most persistent effect of dams is the
alteration to the natural flow regimes that is described in Section 4.
The flood peak is suppressed, and dry season flows increased to even out
the hydrograph downstream. These impacts can extend throughout the length
of the river in the case of a single, mainstream dam. However, the effects
of smaller dams on tributary flows may be local only in the case where
flow in the main river may be sustained by natural flow regimes from other
tributaries. Problems arise when all tributaries are dammed, as the cumulative
effects of many small dams may be as severe as the effects of a major
mainstream dam. Similarly, cascades of dams may completely alter the flow
characteristics of the river on which they are built.
The changes downstream of dams may include (but are not confined to) the
following:
• Alteration of the timing, duration and magnitude of flood peaks and low-flow periods;
• Modification of river temperature that may influence the timing or initiation of reproduction and breeding migrations by aquatic animals;
• Declines in the amounts of FPOM and suspended inorganic material and reductions in the distances over which they are transported;
• Reductions in the extent and duration of floodplain inundation, and modification of the flood-pulse resulting in reductions in land-water exchanges of material.
The
combined effects of dams can depend upon where they are constructed along
the course of a river, and have been described in the discussion of the
Serial Discontinuity Concept (see Section 7). Increasing awareness of
the detrimental effects of dams on river ecology has focussed attention
on the need to release water from dams in a way that mimics the natural
pattern of flood peaks and low flow periods, as well as allocation of
sufficient water to maintain the ecological function of areas downstream.
The latter has given rise to attempts to estimate and maintain the minimum
flows needed to achieve this goal. Even where minimum flows can be estimated
accurately (and this is certainly something that is difficult to calculate),
allocation of sufficient water to ecological functions is a major challenge
for river managers since water allocated to one function (e.g. irrigation)
is usually unavailable for another (e.g. maintaining river health). Furthermore
the economic benefits of increased irrigation, of flood control, or of
hydroelectric power generation are relatively easy to measure, whereas
the benefits of maintaining ecological function are often less tangible.
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