When a few hundred demonstrators, mostly from indigenous communities, temporarily occupied the construction site of the Belo Monte dam on Brazil's Xingu River early on 27 October, workers laid down their tools. But the Brazilian government did not back down from its stance that this hydroelectric project on a tributary of the Amazon — expected to be among the world's largest, with a capacity of 11,000 megawatts, when completed in 2015 — is essential to meeting the energy needs of a booming economy. Under a court order, the demonstrators vacated the site later the same day, but the dam remains the subject of fierce litigation.

The episode briefly drew the world's attention to a controversial mega-project, but this is only part of a larger picture. Led by Brazil, governments in the region are increasingly looking to tap into the Amazon system to slake a growing thirst for energy. If current plans are realized, a wave of dam construction will bring staggering change and development to the rainforest in the coming decades.

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In a global context the Amazon stands out as an area of untapped potential, with the world's greatest river system and a paucity of hydroelectric stations, says Mark Mulligan, a geographer at King's College London, who has led the development of an interactive database of more than 36,000 dams around the world. One of his former students, Leonardo Sáenz, has moved on to Conservation International in Arlington, Virginia, where he is improving the database and incorporating dams that are planned and under construction in the Amazon (see 'A beckoning prize'). The goal is to understand how those investments affect the broader landscape, both physically and economically.

According to the conservation group WWF, less than 10% of Brazil's electrical power comes from dams in the Amazon region at present. The Belo Monte dam would boost this figure, and many more projects are on the drawing board, including 18 dams proposed for the Tapajós tributary system alone over the coming decade. Brazil has also signed an agreement to develop hydroelectric dams in the Peruvian Amazon in exchange for a share of the power.

Although the dams promise carbon-free electricity, they also lead to more road construction and deforestation as well as invasions of migrant workers and massive methane emissions when large swathes of forest are drowned. And, increasingly, experts fear that changing patterns of rainfall brought about by deforestation and climate change could reduce the energy return from dams, rendering many investments obsolete.

"It's really easy to get your infrastructure wrong, and that poses serious investment risks in the long run," says John Matthews, a freshwater expert at Conservation International. "From this perspective climate change presents the ultimate risk in the Amazon." Matthews fears that Brazil could become perilously reliant on an uncertain energy source, even as the government builds more dams.

"They are opening a new hydropower frontier, the last hydropower frontier in South America," says Pedro Bara, who works for the WWF in Brasilia. "In 30 years, if all of the plans were implemented, half of Brazil's energy would come from the Amazon."