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Andes dams could threaten food security for millions in Amazon basin

  • More than 275 hydroelectric projects are planned for the Amazon basin, the majority of which could be constructed in the Andes whose rivers supply over 90 percent of the basin’s sediments and over half its nutrients.
  • A new study projects huge environmental costs for six of these dams, which together will retain 900 million tons of river sediment annually, reducing supplies of phosphorus and nitrogen, and threatening fish populations and soil quality downstream.
  • Accumulating sediments upstream of dams are projected to release 10 million tons of carbon into the atmosphere each year, significantly contributing to global warming, and would contaminate waters and the aquatic life they support with mercury.
  • The construction of these dams should be reconsidered to preserve food security and the livelihoods of millions of people in the Amazon Basin.
The Marañón River in Peru and site of the proposed Pongo de Manseriche dam, which researchers say would be so catastrophically harmful to the environment that it should not be built. Photo by Rocky Contos, used under a CC BY-NC-SA 2.0 license

The Amazon and its tributaries are slated for intense hydroelectric development, aimed at supplying electricity to South America’s 400 million residents and for energy-intensive industries such as mining and smelting. But environmental groups and scientists have raised serious concerns over the huge impact of such dams, built within one of the most biodiverse and productive ecosystems on Earth.

More than 275 dams are currently planned for the Amazon River basin, with most proposed for the Andes Mountains, where steep gorges allow for deep storage reservoirs. But in a paper published in PLoS ONE this August, the Amazon Waters Initiative expert working group predicted that these Andean dams could have severe environmental impacts downstream, affecting the entire Amazon basin, and threatening the livelihoods, diet and health of millions of people.

The international research team combined historical data from Amazonian rivers with mechanistic models to predict the impact of six mega-dams currently proposed or under development in the Andes: four in Peru, the Pongo de Manseriche dam on the Marañón River, the Inambari dam on the Inambari River, TAM 40 on the Ucayali River, and Pongo de Aguirre on the Huallaga River; plus two mega-dams in Bolivia, Agosto del Bala on the Beni River, and Rositas on the Grande River.

Together, these six dams have an expected generating capacity greater than ten gigawatts of electricity. The six are of particular concern because they’re the largest and farthest downstream of any big dams planned for major Andean tributaries.

Scientists looked at potential impacts above and below the planned dams, “including reductions in downstream sediment and nutrient supplies, changes in downstream flood pulse, changes in upstream and downstream fish yields, reservoir siltation, greenhouse gas emissions and mercury contamination.”

They found that these dams will, if constructed, dramatically reduce sediment transport throughout the Amazon watershed, putting the food security of millions of people at risk.

“The environmental impacts of these six dams are so large, multidimensional, and far reaching that their construction cannot be justified,” says lead-author Bruce Forsberg, senior researcher at the National Institute for Amazon Research in Manaus.

In addition, dam construction will have immediate effects on the local environment, flooding forests upstream and dramatically reducing river levels downstream, while displacing indigenous and traditional communities.

Map of the Amazon basin, showing the locations of the six proposed dams. Fig 1. In Forsberg et al. (2017), licensed under a CC By 4.0 license

Sediment flow

Although the Andean Highlands of Peru and Bolivia occupy just 11 percent of the Amazon’s total area, they provide 93 percent of the sediment supplied to the Amazon basin. These critically important sediments carry nutrients vital for aquatic plants and plankton, supplying the bottom-rung of the river food chain as well as fertilizing and replenishing downstream soils during flood season.

The research team calculated that the six new dams would hold back nearly 900 million tons of river sediment annually, preventing them being carried to the flood plains of the Amazon basin and delta.

“The expected reduction of sediment and nutrients beyond the dam sites would be catastrophic for the region’s wildlife as well as countless communities that rely on the river for their agricultural needs,” said Forsberg in a statement.

The new study estimated that the planned dams would cause the Amazon basin to lose 64 percent of its total sediment supply, while also seeing a 51 percent decrease in phosphorus and a 23 percent decrease in nitrogen. Such a massive sediment and nutrient deficit could have profound impacts on downriver ecosystems and human communities.

Phosphorus feeds microscopic aquatic plants called phytoplankton, the primary food source for 40 percent of commercially caught fish in the Amazon basin. Nitrogen is a major component in farm and garden fertilizers, and natural nitrogen transport by the Amazon River and its tributaries is crucial for the vast region’s soil fertility. The dams, say the researchers, are therefore likely to reduce basin and delta fishing yields and increase reliance on chemical fertilizers, both putting food security at risk.

These forecasts aren’t only theoretical. In the case of the Tucurui dam on Brazil’s Tocantins River, a reduced nutrient supply was linked to the collapse of the commercial mapará catfish (Hypophthalmus spp.) fishery. Other fisheries on the river experienced up to 70 percent declines in productivity since the dam was built.

Montane rainforest in Cordillera Azul National Park near the Ucayali River. Forests, biodiversity and local communities can be severely impacted by mega-dams due to reservoir inundation. Photo by A. Del Campo, CC BY 3.0

Dulling the living pulse of the Amazon

Vital sediments and nutrients will not only be blocked behind dams, the new impoundments and dams will control and curtail the Amazon basin’s natural river flow, preventing the annual flooding regimes that support a dynamic mosaic of ecosystems in waterways and across the Amazon basin floodplains.

Whereas seasonal changes in rainfall in the Andes currently drive flooding in the basin, the flow of water through dams would be carefully managed, reducing the gigantic flood pulses that have been seen for millennia, putting ecosystems that are modulated by wet and dry seasons under threat.

None of the six proposed dams have published flow management schemes, so Forsberg and his colleagues instead estimated floodplain impacts based on hydrodynamics recorded historically at the existing Balbina and Tucurui dams. These big Brazilian dams substantially reduce the annual variation in water levels in the Uatumã and Tocantins floodplains, and similar impacts can be expected from the six proposed Andes dams.

Thus in downstream areas that previously experienced periodic seasonal flooding, low-lying terrain may be permanently inundated and upland areas permanently dry. This could have dramatic repercussions on commercial fisheries, ending the genetic mixing that occurs when Amazon streams overflow and merge in flood times, allowing fish to mate. These populations, thanks to the dams, will never have a chance to meet, thus likely reducing genetic diversity and resilience.

Peru’s Madre de Dios River, site of the proposed Inambari dam. The dam is so controversial that it has been put on hold for now, though plans to revive it keep being put forward. Photo by International Rivers, used under a CC BY-NC-SA 2.0 license

Toxic waters

The potential negative impacts enumerated by the study aren’t limited to downstream human communities and ecosystems — the proposed dams would have severe upstream effects as well.

The new reservoir drowns vegetation on the old riverbanks, which release large quantities of greenhouse gases when they die. The decomposing plants starve the water of oxygen, favoring the growth of microbes that convert naturally found mercury in the water into methylmercury, a toxic substance that accumulates in fish and other aquatic organisms, ultimately working its way up the food chain onto the plates of local people. Dams already built in the Amazon are known to contain higher levels of methylmercury in their reservoirs.

The reservoirs not only produce methylmercury microbally, but also accumulate it as a waste product from upstream illegal gold mining operations. The seriousness of this health problem becomes evident when one considers the huge amount of fish Amazon basin residents consume, in a region where a kilo of beef can cost between $15-20 reals, while a kilo of mapará, a popular commercial fish, sells for a mere $3 reals or can be caught for free in the local reservoir or river.

The methylmercury risk is compounded by the fact that vast quantities of nutrients held in the sediments trapped behind dams can also fuel exploding fish populations, so that new fisheries often spring up within and near nutrient-rich dam reservoirs in the tropics. However, local people are often unaware of the methylmercury risk, and bio-accumulation in the human body is slow, so severe health impacts from mercury poisoning may not show up for years, after it is too late.

The Tucurui Dam, on Brazil’s Tocantins River, where reduced nutrient supply caused by the dam was linked to the collapse of the commercial mapará catfish (Hypophthalmus spp.) fishery. Other fisheries on the river experienced up to 70 percent declines in productivity since the dam was built. Photo by International Rivers, used under a CC BY-NC-SA 2.0 license

Reservoirs emit greenhouse gases

Although hydroelectricity is often presented by dam proponents as a clean energy source, it is now understood to produce high levels of greenhouse gas emissions. Globally, hydroelectric reservoirs are associated with 1.3 percent of all man-made greenhouse gas emissions. Tropical reservoirs in particular emit very high levels because rotting vegetation and nutrient-rich waters fuel algal blooms and methane-producing microbes. Methane is some 25 times more potent than carbon as a greenhouse gas.

For four of the proposed dams where data was available, Forsberg and colleagues estimated average daily emissions of 2,890 milligrams of carbon over their first thirty years of life. The worst of the dams evaluated was Peru’s Manseriche Dam, projected to release carbon into the atmosphere at rates similar to an oil-fired power plant.

“The study represents an ecosystem wakeup call for both developers and ecologists to work together across national borders to find solutions that mitigate infrastructure and its potential large-scale impacts at scales unimaginable several decades ago,” said Michael Goulding, senior aquatic scientist at the Wildlife Conservation Society, and study co-author.

As more evidence accumulates regarding the negative effects of dams on people, animals, plants and the atmosphere, attitudes towards hydroelectric power seem to be changing, noted the scientists.

Local opposition to many of the planned Andes dams has been intense, and work on the Inambari Dam stalled in 2011 because of protests, though there have been attempts to revive it since then. Last October, the head of the Ministry of Energy and Mines in Peru, Gonzalo Tamayo, announced that the construction of large hydropower plants in the Amazon jungle is not on their current agenda, but no dams have been officially ruled out, and plans for dam construction may resume if Peru’s energy surplus runs out in the next decade, as forecast.

Peruvian protests against the Inambari dam. Photo by International Rivers, used under a CC BY-NC-SA 2.0 license

Alternative energy sources

Alternative dam designs or relocation may mitigate the impacts of some of the six proposed dams, but not all, say the researchers. The Manseriche Dam, they conclude, would be so environmentally damaging that it should not go ahead in any form, and an alternative energy source should be pursued instead.

“Switching to alternative energy sources like wind and solar is now economically viable and would cause much less impact,” says Forsberg.

The new study is just one of many demonstrating that the true economic, social and environmental costs of hydroelectric power in the Andes headwaters and Amazon basin may be too high, no matter the immense energy-generating capacity of such projects.

However, the jury is still out as to whether Latin American governments — which have a long history of supporting Amazon mega-dams — will be willing to listen to scientists and shift their priorities from hydropower to alternative energy sources.


Forsberg, B. R., Melack, J. M., Dunne, T., Barthem, R. B., Goulding, M., Paiva, R. C., … & Weisser, S. (2017). The potential impact of new Andean dams on Amazon fluvial ecosystems. PloS one12(8), e0182254.

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Submerged vegetation in the reservoir of Brazil’s Balbina dam. Tropical reservoirs emit large amounts of greenhouse gases because rotting vegetation and nutrient-rich waters fuel algal blooms and methane-producing microbes. Methane is some 25 times more potent than carbon as a greenhouse gas. Photo by Seabirds licensed under the Creative Commons Attribution 3.0 Unported license
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