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Calculating hydroelectric plants' environmental impact is not simple. Dam building requires steel and cement, the manufacture of which entails greenhouse gas emissions. Estimates for these emissions are relatively straightforward to make and show that the consequences are small compared to the benefifits of generating greenhouse gas-free hydroelectric power. But more difficult-to-estimate greenhouse gas contributions occur during the plants operation. The submerging of large areas behind dams results in the microbial decomposition of vegetation, which produces substantial quantities of methane—a potent greenhouse gas. Furthermore, eradicating vegetation eliminates the consumption of the greenhouse gas CO2 by that vegetation. Estimating amounts of these gases is difficult, and amounts vary depending upon the specific location, terrain, and power intensity of the dam with its associated lake.

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