Please answer the questions after reading the following excerpt from Silenced
Rivers: The Ecology and Politics of Large Dams by Patrick McCully
Rivers No More: The Environmental Effects of Large Dams
Most of the impacts of river engineering are extremely difficult, and in many cases
impossible, to predict with certainty. Theories on the ecological dynamics of rivers
are mainly based on short-term studies of small temperate watersheds, so there is a
limited understanding of the functioning of large rivers in temperate regions — or
of rivers of any size in the tropics. Most of the major rivers in Europe and the
United States were dyked, straightened, dredged and dammed long before their
ecology or hydrology had been seriously studied. In the tropics, where research
funds are few, often the only scientific study of a river system has been done to
find where best to dam it.
As every river is unique in terms of its flow patterns, the landscapes it flows
through and the species it supports, so the design and operating pattern of every
dam is unique, as are the effects the dam has on the river and its associated
ecosystems. While the great majority of the world's large dams and all of the major
dams have been completed within the last six decades, some of the environmental
effects of a dam may not be realised for hundreds of years after construction. A
dam can thus be regarded as a huge, long-term and largely irreversible
environmental experiment without a control.
The two main categories of environmental impacts of dams are those which
are inherent to dam construction and those which are due to the specific mode of
operation of each dam. The most significant consequence of this myriad of
complex and interconnected environmental disruptions is that they tend to fragment
the riverine ecosystem, isolating populations of species living up and downstream
of the dam and cutting off migrations and other species movements. Because
almost all dams reduce normal flooding, they also fragment ecosystems by
isolating the river from its floodplain, turning what fish biologists term a
'floodplain river' into a 'reservoir river'. The elimination of the benefits provided by
natural flooding may be the single most ecologically damaging impact of a dam.
This fragmentation of river ecosystems has undoubtedly resulted in a massive
reduction in the number of species in the world's watersheds.
Some of the environmental effects of dams can benefit some species. For
example, impounding a reservoir will create habitat for lake fish and warm water
released from a reservoir can increase the abundance of species of fish which failed
to thrive in the cool river. But because dams alter the conditions to which local
ecosystems have adapted, the overall impact of a dam will almost without
exception be to reduce species diversity.
No one has yet managed to assess with any accuracy the global extent of the
fragmentation of river ecosystems by dams and water diversions. Two Swedish
ecologists, however, have estimated the degree of damage to river systems in the
US, Canada, Europe and the former USSR. Mats Dynesius and Christer Nilsson of
the University of Umeå found that fully 77 per cent of the total water discharge of
the 139 largest river systems in these countries is 'strongly or moderately affected
by fragmentation of the river channels by dams and by water regulation resulting
from reservoir operation, interbasin diversion and irrigation'. 'As a result of habitat
destruction and obstruction to organism dispersal,' Dynesius and Nilsson conclude,
'many riverine species may have become extinct over vast areas, whereas
populations of others have become fragmented and run the risk of future
extinction.'
The permanent inundation of forests, wetlands and wildlife is perhaps the
most obvious ecological effect of a dam. Reservoirs have flooded vast areas — at
least 400,000 square kilometres have been lost worldwide. Yet it is not only the
amount of land lost which is important, but also its quality: river and floodplain
habitats are some of the world's most diverse ecosystems. Plants and animals which
are closely adapted to valley bottom habitats can often not survive along the edge
of a reservoir. Dams also tend to be built in remote areas which are the last refuge
for species which have been displaced by development in other regions. No one has
any idea how many species of plants and animals are now extinct because their last
habitat was flooded by a dam but the number is likely far from negligible. As well
as destroying habitat, reservoirs can also cut off migratory routes across the valley
and along the river. Because it isolates populations, this ecosystem fragmentation
also leads to the risks of inbreeding from a smaller genetic pool.
The five-dam Mahaweli megascheme in Sri Lanka, the main purpose of
which is to expand irrigation in previously forested areas, has submerged and
turned into agricultural land the habitat of seven endangered and two threatened
animal species, the purple-faced langur and the toque macaque, both of which only
occur on the island. One of the endangered species is the elephant, 800 of which
lived in the project area. An important migratory route for the elephants has been
cut off by reservoirs and canals, and the animals have now become a dangerous
pest for the farmers who have been brought into the area, reducing the survival
chances of the remaining animals.
It is often not just the forests within the reservoir area, around the dam site
and transmission lines and in the areas slated to be converted to agriculture which
are lost when a dam is built in a forested area. In many cases farmers displaced by
a reservoir have had to clear forests further up the sides of the valley to grow their
crops and build new homes. The access to previously remote areas allowed by new
roads and reservoirs can also accelerate deforestation: every large dam which has
been built in a forest area in Thailand has attracted loggers as well as developers
who have built golf courses and resorts along the edges of reservoirs.
The number of fish species which thrive in the relatively uniform habitats
created by reservoirs is only a tiny fraction of the number which have evolved in
the diverse niches provided by rivers. Because few areas have economically
valuable fish adapted to the still waters of an artificial lake, fishery departments
across the world have introduced into reservoirs a handful of species — mainly
types of tilapia and carp in the tropics and trout, bass and catfish in temperate
regions — which can be reared in hatcheries and can support reservoir fisheries.
These introductions, which compete with those native species which persist in the
reservoir, and also spread far upstream and downstream of the dam, have greatly
magnified the effects of dams and diversions in hastening the decline and
extinction of fish species around the world.
As well as flooding and fragmenting some of the world's best wildlife
habitats, reservoirs have also inundated some of the world most beautiful and
spectacular river scenery. Probably the greatest loss of the planet's scenic heritage
to a reservoir was the inundation of the spectacular Sete Quedas waterfall at Guáira
on the Brazilian-Paraguayan border, now just a rock formation at the bottom of
Itaipú Reservoir. At Guáira the mighty Paraná suddenly narrowed to a width of just
60 metres — less than a tenth as wide as the Horseshoe Falls at Niagara — and
then thundered over 18 separate cataracts each more than 30 metres high. More
water surged and boiled through the rocks and whirlpools of Sete Quedas than any
other waterfall in the world — about half as much again as over both falls at
Niagara combined. 'A more imposing spectacle can scarcely be conceived,' a 19th
century French traveller wrote of Sete Quedas.
1. According to the article, is it possible to predict the impacts of river
engineering accurately? Why?
2. In the tropics, why don’t scientists do more research on a river system
before they dam it?
3. What are the two major kinds of environmental impacts of a dam?
4. What is probably the most damaging ecological impact of a dam?
5. How would dams influence the variety (kinds) of species?
6. Does the author have a positive or negative view of a dam? How do you
know?
7. What kind of environmental impact of a dam can be seen easily?
8. What do two Swedish ecologists, Mats Dynesius and Christer Nilsson,
predict about the riverine species?
9. Why does the elephant become one of the endangered species in Sri
Lanka?
10. What is the result of the introduction of a handful of species such as tilapia
and carp in the tropics and trout, bass, and catfish in temperate regions?