Rivers No More: The Environmental Effects of Large Dams

Date: 
Monday, January 1, 2001

An excerpt from Silenced Rivers: The Ecology and Politics of Large Dams, by Patrick McCully

Flooding for Posterity

"We're going to save all this for posterity. We're going to cover it up with water so that no one can disturb it."
- Comment by Brazilian dam engineer viewing scenic stretch of river to be flooded by Cachoeira Porteira Dam, 1984

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.

Read the next section of this chapter, Dams and Geology

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