Hydroelectric dams increase methane emissions  Hydroelectric dams emit a lot of methane, which is an even more potent greenhouse gas then carbon dioxide. This happens when the plants and vegetation submerged in the reservoir start to rot under water: they then produce methane which bubbles up and is released into the atmosphere. On balance, some dams produce more greenhouse gasses than conventional power plants running on fossil fuel. [1]  [1] New Scientist, ‘Hydroelectric power's dirty secret revealed’, 2005