Which is the most negative environmental consequence of building dams for hydroelectric power?

Hydroelectric projects are built to generate electricity by impounding the flowing water in reservoirs (known as impounding reservoirs). There are a number of negative effects associated with such projects, including submerging large tracts of land, the deposition of sediments, the loss of sediments from the stream, and the displacement of people and animals, among others.

In my opinion, the biggest environmental issue with the dams is the generation of greenhouse gases. A large number of plants are submerged in the reservoir and a lot of organic matter (that would otherwise have been carried downstream) is also collected over there. The decomposition of all this matter consumes the dissolved oxygen of the water and ultimately results in anaerobic decomposition. During such type of decomposition, greenhouse gases (such as methane) are generated. These greenhouse gases are directly responsible for global warming.

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