Destructive dams

Aerial photograph of the Balbina dam archipelago Aerial photograph of the Balbina dam archipelago Eduardo M. Venticinque
01 Jul
They may enjoy a clean, green reputation, but the environmental impact of dams built for hydroelectric energy appears far worse than first believed

In the fight to combat climate change, they are generally seen as close and valuable allies. More than 945,000 dams (higher than 15m) have been constructed around the world to date, affecting over 50 per cent of major rivers. The USA built them by the thousands during the last century, and many developing countries are now following suit. An estimated 2,215 dams are planned for South America in the next few years, covering ten million hectares of Brazilian Amazonia in the process.

However, more recently the USA has for years been investing heavily in demolishing it’s dam infrastructure. Stating: ‘While dams can benefit society, they also cause considerable harm to rivers. Dams have depleted fisheries, degraded river ecosystems, and diminished recreational opportunities on nearly all of the nation’s rivers,’ American Rivers, a charity protecting wild rivers in the US, says that nearly 850 dams have been demolished in the USA within the past 20 years.

‘For a long time we’ve known that large-scale hydropower development affects the aquatic ecosystems and biodiversity. But no one realised how much of an impact it also has on the terrestrial ecosystem,’ Carlos Peres, Professor of Conservation Ecology at the School of Environmental Sciences, University of East Anglia, tells Geographical.

Peres, an author on the new PLOS ONE report entitled Widespread Forest Vertebrate Extinctions Induced by a Mega Hydroelectric Dam in Lowland Amazonia, led a team researching the environmental impacts of the Balbina Hydroelectric Dam, opened in north-west Brazil in October 1986.


The Balbina dam created a reservoir area of 443,700 hectares, submerging the forest habitat to create 3,546 islands, the last refuge for the rainforest wildlife displaced by the new reservoir. A 940,000-hectare protected area, the Reserva Biológica (REBIO) do Uatumã, was designated around the site of the dam, with the intentions of offsetting the damage done to the environment by the flooding caused by the reservoir.

‘Obviously all the forest habitat and all the populations in the areas that were flooded are essentially dead,’ says Peres. ‘What we show is that even the populations that are still in the persisting forested areas gradually bleed away. You have a gradual process of local extinctions, to the point where we’ve now lost over 70 per cent of all the populations that were stranded in those islands.’

The study shows how key vertebrate species (such as armadillos, anteaters and pumas) have suffered from their dramatic loss in habitat. Limiting their territories in such an extreme way made it impossible for them to meet their basic ecological needs. Ultimately, the report estimates that only 25 islands – 0.7 per cent of the 3,546 total number – are now home to ‘a species-rich vertebrate assemblage’ that contains at least 80 per cent of the original species .

‘You lose precisely the most important species, the species that we are most concerned about in terms of conservation,’ continues Peres. ‘The species that tend to survive are the very resilient ones, those that basically survive anywhere. The species that do survive are not the species that we are particularly concerned about.’

anteaterAnteater recorded during surveys of the Balbina archipelago (Image: Eduardo M. Venticinque)

Could the negative impacts found by this study be because of the Balbina dam’s location, in an area of the rainforest suffering from other environmental problems, such as deforestation?

‘No, on the contrary,’ replies Peres. ‘Our study area in Balbina – that massive artificial archipelago formed by the hydroelectric dam – is also the largest biological reserve in Brazil. Therefore, the results that you are seeing are very conservative. They clearly underestimate what dams do to biodiversity in most other places.’

In Peres’ words, Balbina was ‘a recipe for how not to build a dam’. He stresses the need for countries such as Brazil to stop investing so heavily in the construction of these infrastructure projects, while there is still time to prevent further damage from being done.

‘I think that there has to be some official policy recognition that these kinds of biodiversity impacts have to be taken into account in the initial environmental impact assessments (EIA) on which the approval of a new dam is based,’ he concludes.

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