Geoscience Reference
In-Depth Information
during the dry season. When the rains start, or floods
occur, there is likely to be major movement of sediment
which would normally be transported in the river. When
a river flows into a new lake, its velocity will fall and so
the suspended sediments will be deposited in the lake.
Some of the dams on the Huang Ho in China have the
unenviable global reputation of being filled most rapidly
by sediment. The catchment area of the Huang Ho
includes the loess plateau of western China, which is easily
eroded during the rainy season. The river used to be
known as the Yellow River because its sediment load
discoloured the water. The Sanmenxia dam on the Huang
Ho began impounding water in 1960. Within seven and
a half years the reservoir had lost 35 per cent of its capacity,
with estimates of 3,391 M m
3
of silt deposited in that time!
The lake is now filled with sediment, with only 10 per cent
of its original capacity, and can generate electricity for only
a few months in winter. Silt continues to pit the turbine
blades and shorten their working life. All dams suffer this
problem to varying degrees. The life of the dam is also
important in economic terms. If power is generated for
only, say, forty years, instead of the predicted 100 years, it
will be much more expensive per kilowatt of electricity
generated.
Evaporation
. When water is stored at the surface it will
evaporate. In dryland areas this is a particular problem,
as rates of evaporation are high because of the dry
atmosphere, high inputs of solar energy and temperature
levels. In Lake Nasser, behind the Aswan High Dam, it is
estimated, about 10 G m
3
of water are lost each year
through evaporation. Although much of this evaporated
water may have previously run off direct to the sea, it is
nevertheless an important loss of water resources.
Unfortunately there is little that can be done to reduce it.
The water surface can be covered with chemicals or
artificial skins but that would be uneconomic for large
lakes.
Environmental and ecological changes.
The removal of
sediment from the river into the lake means that the water
released from the dam is relatively sediment-free. As we
saw in
Chapter 14,
if a river loses sediment it has more
energy available for erosion. Construction of the
Danjiangkou dam on a tributary of the Yangtze led to
degradation of the river bed and banks up to 500 km
below the dam. Reduction of silt load in the Hwang Ho
through damming has reduced siltation and increased
scouring in its lower course and reduced the risk of
flooding, though the effect is only short-term. Similar
effects have been noticed on the Nile. The lack of annual
sediment accumulation has caused rapid retreat of the
delta coastline, with consequences for the coastal fishing
industry. The loss of silt reduces the natural fertilizing
effect of flooding, so, to sustain yields, artificial fertilizers
have to be used.
Water tables are affected by dam construction, leading
to waterlogging of the immediate surrounds and the
potential for salinization if there is insufficient downward
movement of water. Along the coast, reduced freshwater
flow can lead to an incursion of salt water into the water
table.
The new lake will cover and destroy all existing
vegetation. Trees may be left to decay, releasing methane,
and habitats will be lost. Such environmental impact is
increasingly causing concern over major dam construc-
tion such as the Three Gorges project in China. Thorough
surveys of the impact of a dam are not always made.
The Rasi Salai dam in eastern Thailand was built on a huge
salt dome, so the water soon became too saline for
irrigation use.
An interesting biological consequence of the increase
of freshwater surface has been the encroachment of water
weeds. The water hyacinth is a major problem. Within two
years of construction, 50 per cent of the surface area of a
lake in Surinam (South America) was covered by this
plant, and Lake Kariba has experienced water-fern
encroachment.
Pests and diseases
. Bodies of still water provide an
attractive environment for many pests and diseases in
dryland areas. Malaria can be an increased problem,
though in smaller dams changing water levels may strand
larvae. Bilharzia is another water-related disease which has
increased near major dam projects. People contract the
disease through bathing, fishing, washing clothes or
collecting water from areas infected by the parasitic larvae.
After the Volta dam in Ghana was constructed the
incidence of bilharzia in children under ten had risen to
90 per cent. Positive effects can occur. River blindness is
caused by a fly which breeds in fast-flowing sections of
rivers. As some of these habitats have disappeared under
the reservoirs, so the incidence of the disease has declined.
Management
. Many dam schemes have been less
successful than expected because of poor management
and maintenance following construction. Frequently too
much attention is paid to the design and construction of
the project, little training being given to sustain the
management of the water. Bureaucracy, politics and
complex administration can exacerbate the difficulties.
Major problems may arise where water released from
dams flows into other countries. Although some co-
operation may occur over the supply and use of water
resources the country possessing the dam will have most
control over how much water is released and when. Turkey
