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Dam-building spree pushes Amazon Basin’s aquatic life closer to extinction

  • A recent study looking at 343,000 kilometers of waterways along 6,000 rivers across the Amazon Basin highlights the importance of free-flowing rivers to migratory species of fish and turtles, as well as river dolphins.
  • More than 20 species are threatened by the construction of hydroelectric dams on long-distance rivers, considered to be key corridors for biodiversity.
  • The study’s researchers identified 434 dams that have either been built or are currently under construction across the whole of the Amazon Basin, and a further 463 proposed dams that are in various stages of planning.
  • One of the proposed dams on the Tapajós River would leave at least four pods of river dolphins isolated from one another, leaving them facing a similar fate to the tucuxi river dolphins impacted by similar projects on the Madeira River and today threatened with extinction.

“People need to think about river basins or freshwater systems like a heart, a living system,” says Bernardo Caldas, from WWF-Brasil.

Caldas is the co-lead author of a new study investigating how freshwater systems in the Amazon Basin are interconnected. It’s the first study of its kind to combine two layers of analysis of 6,000 rivers from across the Amazon Basin, which together make up some 343,000 kilometers (213,000 miles) of waterways. On one level, the study details the current level of connectivity of these rivers, as well as pointing to how this could change in the future if all the hydroelectric dams currently planned for the region are built. The second layer of analysis looks at the presence of migratory fish, turtle and dolphin species in the same rivers, detailing the importance of free-flowing rivers whose course is uninterrupted by hydropower dams.

Furthermore, the study examines how the rivers of the Amazon Basin are connected to lakes formed by flooding during the rainy season and play an important role in the life cycles of the species focused on in the study. “It is these more sensitive environments, such as the floodplains, that end up being most affected by the hydroelectric dams,” Caldas says.

The rhythms of the waterways of the Amazon Basin, marked by low- and high-water periods, are vital to the health of the aquatic ecosystem, yet have been disturbed by a number of factors, such as a changing climate, deforestation, changes in land use, uncontrolled and disorderly urbanization, the expansion of roads, and the construction of dams.

In the case of the dams, their presence impacts the rhythm of flow of the rivers and harms the life cycles and movements of the animals living both upstream and downstream. The dams reduce the levels of nutrient-rich sediments in the river and also impact the lives of the people who live in the rainforest — not only through increased levels of flooding, but also through impacts on fish populations, one of the main sources of nutrition and subsistence for communities in the basin. One stark example is the fall in fish stocks in the Volta Grande bend of the Xingu River following the construction of the Belo Monte hydropower complex upstream.

The Jirau hydropower dam in the Brazilian state of Rondônia poses a risk to the river dolphins of the Madeira River. Image courtesy of the Growth Acceleration Program.

Water and wildlife movements restricted

One of the aims of this study was to map the degree of connectedness of the waterways of the Amazon Basin and the relevance of this network to the species that live there. The study’s researchers analyzed 44 species of migratory fish, every species of river dolphin, and one species of turtle, the Arrau turtle (Podocnemis expansa), the only one known to migrate more than 500 km (300 mi). Long-distance migratory species are vital to understanding the need to have connected (i.e. dam-free) rivers across the water basin.

Current distribution across the Amazon Basin of (a) fish species, (b) Arrau turtle, and (c) river dolphins analyzed by the study. Image courtesy of Caldas et al. (2023).

The Madeira River, a major tributary of the Amazon, is an emblematic example of habitat fragmentation, where one population of tucuxi river dolphins (Sotalia fluviatilis) was confined between the Santo Antônio and Jirau hydropower dams. The construction of the dams also impacted migratory fish species and led to a decrease in fish populations, as in the case of the gilded catfish (Brachyplatystoma rousseauxii). Indigenous towns and villages in the surrounding areas have also been hit hard by more intense and more frequent flooding since the construction of the dams.

“Today, if we look at the energy mix mainly in Brazil, which covers the largest part of the Amazon and is home to the rivers with the greatest potential for hydroelectric power generation, we can see that there has been a rapid increase [in the number of dams],” says study co-author Mariana Paschoalini Frias, from the Federal University of Juiz de Fora. The authors identified 434 dams that have either been built or are currently under construction across the whole of the Amazon Basin, and a further 463 proposed dams that are in various stages of planning.

Existing dam projects, in yellow, and potential ones, in magenta, throughout the Amazon Basin. Image courtesy of Caldas et al. (2023).

In the worst-case scenario, in which all the dams currently proposed are built, 18 of the 98 free-flowing rivers classified by the study as either long or very long in length (defined as more than 500 km) would lose their status as freshwater connectivity corridors. This includes the Amazon River itself, which the study highlights as being one of the most important rivers on which to maintain current connectivity levels.

“This surprised us, because there are no hydroelectric projects on the Amazon River, but the system would become so saturated and so compromised that even the Amazon River itself would lose its dynamism and important functions within the basin as a whole,” Caldas says.

The map at the top shows current levels of connectivity of the rivers of the Amazon Basin. The map at the bottom shows how that connectivity might look if all of the currently proposed hydropower projects in the region are built. The darker the shade of red, the greater the impact on connectivity. Image courtesy of Caldas et al. (2023).

“If all the hydroelectric dams that are planned for the Amazon region are built by 2030, we’re going to lose almost all connectivity,” Paschoalini says. “Across every one of the rivers, the line starts to turn, if not red, then orange. And if not orange, then a little pinker. It goes on decreasing in the connectivity index, showing how much we are losing.”

According to the study, more than 20 species of long-distance migratory animals, as well as river dolphins, would be threatened by the construction of hydropower dams along the Amazon, Negro, Maranhão, Ucayali, Preto do Igapó-Açu, Beni and Uraricoera rivers. Every one of these is considered a priority corridor for biodiversity.

Paschoalini highlights the numerous hydropower projects planned for the Tapajós River as being particularly dire: “If these hydroelectric dams were built, we would end up isolating at least four groups of river dolphins, for example.” Another case is the binational hydropower project on the Mamoré River on the Brazil-Bolivia border, which would threaten the connectivity of the Amazon Basin between the two countries. “This would increase the fragmentation of the river dolphin population in the region and would also see the situation worsen for large migratory catfish,” Paschoalini says.

A tucuxi river dolphin, one of several species whose existence is threatened by the proliferation of dams across the Amazon Basin. Image courtesy of Jéssica Melo.

Rethinking the energy mix

Experts note that although many free-flowing rivers are located within protected areas, the water itself isn’t subject to special protections. “We don’t have many mechanisms that help us to protect the rivers’ dynamics,” Caldas says. He calls for a more expansive diagnosis of the situation regarding the waterways in order to shape management policy and decision-making.

Data from mapping initiative MapBiomas Água shows a decline in the water surface area between 2001 and 2021 across the Brazilian Amazon, compared to the historical average since 1985. The most likely causes for this trend are climate change and deforestation.

“The reduction of the water surface area affects aquatic ecosystems and their biodiversity levels,” says Carlos Souza Jr., a technical and scientific coordinator with MapBiomas. “The chemical composition of the water is modified and the isolation of water bodies restricts the flow of aquatic fauna. This isolation is also made worse by large and small dams, which change the flow of water and fauna.”

Souza and fellow researchers previously documented the impacts on the surface water of the Amazon from the development of infrastructure, including as hydropower dams, in a 2019 study.

Experts have also called for a rethink of Brazil’s energy mix, as well as improving the performance of existing hydropower dams and prioritizing wind and solar power. However, there’s strong support from some sectors to keep building new dams; this was demonstrated in a recent speech by Aloísio Vasconcelos, former president of state-owned power utility Eletrobrás, at an event at the National Confederation of Industry (CNI), in which he spoke in favor of the construction of the series of hydropower plants on the Tapajós River.

“The Tapajós River is one of the last remaining free-flowing large rivers in the world,” says Alexandre Gross, an infrastructure and conservation specialist with WWF-Brasil. “In Europe there is currently a trend of deconstructing dams and recovering the free flow of rivers. They’re already in a phase where they are taking down the dams. We are still trying to avoid them being built.”

According to Gross, since the construction of the Itaipu hydropower plant in 1984, the idea has taken hold in Brazil that large-scale dams are necessary.

“It paints hydroelectric energy as a clean form of energy, and this is something that we have to break with. It is not a clean form of energy, given all the social-environmental consequences that it generates.” he says. “The Lula government has said that, if necessary, it would build another Belo Monte. This scares us.”

Banner image of Arrau turtles, courtesy of Camila Ferrara.

This story was reported by Mongabay’s Brazil team and first published here on our Brazil site on June 7, 2023.


Caldas, B., Thieme, M. L., Shahbol, N., Coelho, M. E., Grill, G., Van Damme, P. A., … De Brito Ribeiro, M. C. (2022). Identifying the current and future status of freshwater connectivity corridors in the Amazon Basin. Conservation Science and Practice5(1). doi:10.1111/csp2.12853

Souza Jr., C. M., Kirchhoff, F. T., Oliveira, B. C., Ribeiro, J. G., & Sales, M. H. (2019). Long-term annual surface water change in the Brazilian Amazon biome: Potential links with deforestation, infrastructure development and climate change. Water11(3), 566. doi:10.3390/w11030566

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