Conservation news

Andes dams twice as numerous as thought are fragmenting the Amazon

The Marañón River flows though the Peruvian Andes, and is a major Amazon River tributary. The Peruvian government is planning to build several major dams on the river, though the projects are fiercely opposed by many local riverine communities. Photo by Gato Montes on Wikimedia CC-BY 3.0

The scale of hydroelectric development in the Andean Amazon is far more extensive than previously thought, with numerous headwater dams fragmenting river habitats, disrupting natural systems, and affecting the lives and livelihoods of 30 million downstream Amazon basin inhabitants, according to a new study published in Science Advances.

If proposed dams in the region go ahead, sediment transport from the Andes to the Amazon floodplains will cease and migratory routes of freshwater fish will be blocked, threatening food security for downstream communities.

An international team of researchers led by Elizabeth Anderson, a freshwater ecologist at Florida International University in Miami, used satellite imagery to verify reported locations of existing dams in the Amazonian Andes of Colombia, Ecuador, Peru and Bolivia, and to quantify their impact on river connectivity.

The scientists identified 142 dams currently in operation or under construction – twice the number previously estimated. This study represents “the most detailed accounting of dams in the Andean region,” says Kirk Winemiller, an aquatic ecologist at Texas A&M University, who was not involved in the study. The team also included the impact of proposed dams in their model – an additional 160 projects – and found that they would further reduce connectivity on five out of eight major Andean rivers that flow into the Amazon basin, with the Napo, Marañón, Ucayali, Beni, and Mamoré most effected.

Aerial view of the Rio Solimóes flowing through Amazon rainforest in Brazil. In the rainy season, rivers jump their banks and flow into floodplain forests, enriching soils with vital nutrients. Construction of all the planned Andes dams would prevent those annual flooding events. Photo by Iubasi on Wikimedia CC-BY 2.0
Satellite image of a flooded forest in Pará state, Brazil. Dams disrupt flow and flooding regimes vital to aquatic and terrestrial wildlife and plants. Photo courtesy of NASA

The last large-scale survey of hydroelectric development in the Andean Amazon was published in 2012, but hydroelectric development has flourished there in intervening years, and global and regional reports often exclude small hydroelectric projects – estimated to outnumber large hydropower plants 11 to 1, according to another study published last month – meaning that other research using past published data has seriously underestimated the number and impact of dams in the region.

Anderson and colleagues looked at rivers and tributaries in seven Andean Amazon basins. They combined satellite imagery with government planning and energy authority records, and calculated dam impacts using the Dendritic Connectivity Index – a measure of how passable each stretch of river is for fish. They found that the tributary networks supplying the Marañón and Ucayali rivers in Peru (important Amazon River headwater streams) have already lost 20 percent of their original connectivity.

The Andes represents only a tiny fraction of the Amazon basin, but it exerts a disproportionate effect on ecological processes downstream – influencing fish migration and transporting nutrient-rich sediments all the way to the vast floodplains of Brazil and to the Amazon River estuary.

The problem arises because most hydroelectric projects in the Andes Amazon store or divert water for later energy generation, rather than allowing normal river flow to generate electricity continuously. As a result, headwaters dams trap up to 100 percent of the sediments previously carried by the streams, Anderson explains.

A Shawi fisherman on the Rio Paranapura. The new study raises an alarm over the number of existing, under construction and proposed dams in the Andean Amazon, which would seriously diminish connectivity and impact fisheries. Photo by Alvaro Del Campo

Another problem: the dams disrupt the seasonal cycle of flooding in the Amazon basin, reducing or preventing forest inundation, and influencing migration, mating and feeding patterns, as well as social behaviors of both aquatic and terrestrial animals. Fragmentation of the Andes headwaters has “huge ramifications for not only the rivers of the Andean region of the Amazon, but also for the ecology of the entire Amazon basin,” says Winemiller.

Most hydropower development to date has affected the tributary networks of Andean Amazon river main stems, but this may be about to change. Dams are currently planned for the main stems of five out of eight major Andean Amazon rivers. Anderson’s team warns that these projects could result in connectivity reductions on the Marañón, Ucayali, and Beni rivers of more than 50 percent, with the Madre de Dios and Mamoré rivers suffering connectivity decreases of over 35 percent.

That loss in aquatic connectivity could have a big impact on the biodiversity of Amazon fisheries. The team compared their data on river connectivity with a list of freshwater fish species collated from the published literature by the Amazon Fish Project, and found that sites greater than 500 meters above sea level in the Andean Amazon are home to 671 species of freshwater fish – the first published estimate for the Andean headwaters.

Of the four freshwater ecoregions defined for the Andean Amazon, the Amazonas High Andes – spanning Bolivia, Peru, Colombia and Ecuador – has experienced the most intense hydropower development so far, and is also the site of the greatest number of proposed dams. However, the region is likewise home to hundreds of fish species, nearly 40 percent of which are thought to be unique to the region.

The fast-flowing waters of the Topo River in Ecuador. Many Andes fish species are narrowly adapted to inhabit rapids at specific altitudes. Dams would wreak havoc on connectivity and impact fish and fisheries in negative ways. Photo by Elizabeth Anderson

These fish species are specially adapted to steep mountain rapids, with the Amazonas High Andes harboring distinctive collections of species at different elevations. “It is not uncommon for a fish species to just be found in a small part of one basin, and nowhere else,” says Anderson.

Others migrate thousands of miles to spawn or feed. The goliath catfish (Brachyplatystoma sp.), for example, makes the longest freshwater migration in the world, covering almost the entire length of the Amazon River. Proposed tributary and main stem dams could block crucial migration routes for numerous Amazonian fish, as well as altering environmental cues such as flow pulses and flooding that are used by fish as signals to begin migration.

The dams would affect more than fish: the “rhythms of life of many human populations across the Amazon are linked to river flows,” says Anderson, with reduced connectivity set to disrupt seasonal activities like farming, fishing and transporting goods.

Cooperative international water resource management will be critical to protecting the natural and human communities that rely heavily for survival on the flow of Andes and Amazon basin rivers, says Anderson. The new study calls for transboundary assessments of the physical, chemical and biological impacts of Andean dams on the Amazon region, echoing concerns expressed by scientists and environmentalists over hydropower development globally.

“Proposed dams should be required to complete cumulative effects assessments at a basin-scale,” Anderson asserts, and take into account synergistic effects of existing dams. If ratified by Amazonian countries, she adds, the UN Watercourses Convention could provide a legal basis for encouraging more sustainable transnational water management.

Satellite view of the confluence of the Chimore, Ichilo and Mamore rivers in Bolivia. Scientists are urging Amazon nations to take a basin-wide management approach to the planning and construction of dams in the region, especially looking at the cumulative environmental impacts of multiple dams. Photo courtesy of NASA

The newly published study “demonstrates the feasibility of environmental impact assessment at large spatial scales,” says Winemiller, and helps pave the way for similar assessments to be incorporated in large scale, transnational water management planning across South America.

However, to achieve that broad goal, Anderson says, government will need to begin seeing free-flowing rivers as vital and worth protecting. “The future of Andes Amazon connectivity depends on a shift in mindset towards recognition of free-flowing rivers as objects of conservation and then ensuring them adequate protection,” she says.

The political tide may already be turning. In 2014, Colombia announced plans to fully protect the Bita River, an Orinoco River tributary, and the country’s first conserved river. And despite a wave of environmental deregulation initiatives promoting Amazon development, Brazil’s President Michel Temer recently announced a shift in policy away from mega-dams.

“I am hopeful that we will soon see other Andean Amazon countries turn towards rivers as a new frontier for ecosystem conservation,” says Anderson.


Anderson, E. P., Jenkins, C. N.; Heilpern, S., Maldonado-Ocampo, J.A, et al, (2018), Fragmentation of Andes-to-Amazon connectivity by hydropower dams, Science Advances, Vol 4, no. 1 easo1642; DOI: 10.1126/sciadv.aao1642

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Members of the La Roya indigenous community paddle at sunset on the Ucayali River. Dams disrupt fisheries, with major detrimental impacts on indigenous lives and livelihoods. The Andes dams could ultimately impact up to 30 million people. Photo credit: CIFOR on Visualhunt / CC BY-NC-ND