Paris - Building the 428 hydroelectric dams proposed for Brazil's Amazon basin - three times the existing number - would devastate the environment and could even disrupt regional weather, an international team of researchers warned on Wednesday.
Writing in the journal Nature, the scientists unveiled a Dam Environmental Vulnerability Index (DEVI) to score the impact of these future dams across a broad range of criteria and help policy planners decide which ones to leave on the drawing board.
"We have to put the risks of the table and change the way people are looking at the problem," said lead author Edgardo Latrubesse, a professor at the University of Texas at Austin.
"We are massively destroying our natural resources, and time urges us to find some rational alternatives for preservation and sustainable development."
The network of rivers flowing into the Amazon River is the largest in the world, and nourishes the planet's highest concentration of biodiversity.
Damming these enormous tributaries - sometimes dozens of times - blocks nutrients that nourish downstream ecosystems, floods huge tracts of forest, and threatens both aquatic and terrestrial wildlife.
The new index grades dams from 1 (benign) to 100 (highly destructive), and can also be applied to existing structures.
Two huge dams, for example, recently built on the Madeira River, home to the most diverse fish population in the Amazon system, earned alarmingly high scores due to the potential for erosion, runoff pollution, and trapped sediment.
An additional 25 dams on the Madeira are planned for construction further upstream.
"Dams are examples of great engineering achievements for supporting healthy societies and human development, yet spawn massive and negative environmental impacts," said James Syvitski, a top expert in Earth surface dynamics at the University of Colorado-Boulder.
A more comprehensive approach to predicting those impacts is welcome because existing methods fall short, he said, commenting on the new study, to which he did not contribute.
The loss of sediment downstream, for example, is an oft overlooked problem, especially for densely-populated delta regions.
Resculpting Earth's surface
The steady accumulation of silt is essential for maintaining healthy mangroves, the coastal forests that thrive in brackish water, protecting against storm surges and serving as aquatic nurseries for dozens of species.
The loss of sediment is also causing the world's major deltas - home to 600 million people - to sink, part of a triple threat that includes rising seas (driven by climate change) and subsiding land (due to the depletion of underground aquifers).
Earlier research has also shown that changes in sediment flow from the Amazon basin can affect rainfall and storm patterns as far away as the Gulf of Mexico.
"If all the planned dams in the basin are constructed, their cumulative effect will trigger a change in sediment flowing into the Atlantic Ocean that may hinder the regional climate," Latrubesse said.
Worldwide, there are more than 58 500 large dams at least 15 metres from foundation to crest, or impounding at least three million cubic metres.
"We are sculpting the surface of the Earth," Syvitski said.
"Thousands of dams built since the mid-19th century have completely altered the planet's terrestrial plumbing."
China alone has 40% of the world's large dams.
Add those in the US and India, and that percentage rises to two-thirds, according to the International Commission on Large Dams.