Far from being eco-friendly, hydroelectric power can actually cause more greenhouse gases to be released into the atmosphere than coal-fired power plants, according to a report by the World Commission on Dams, a group of scientists, engineers, and enviros supported by the World Conservation Union and the World Bank. The reservoirs created by dams trap organic matter and decaying vegetation, which emit large amounts of greenhouse gases, particularly methane. Emissions can vary widely between hydroelectric projects, but one example is the Balbina reservoir in Brazil, which releases the equivalent of some 3 million tons of carbon dioxide annually, whereas a coal-fired plant producing as much power as the dam at Balbina would release only 350,000 tons a year.