Three Gorges Dam, photo: Harvey Barrison/CC BY-SA
From an environmental perspective hydropower is a mixed bag. Depending on how it's done (micro, small, run of river, large scale) it can be anything from a great low-carbon sustainable energy source to an environmental disaster destroying habitat, displacing people and animals, and even spewing greenhouse gases from the large ponds created behind massive dams. At least that's the conventional eco-wisdom.
When it comes to the greenhouse gases emitted from hydropower reservoirs though, some new research published in Nature Geoscience shows that concern may be overblown. This analysis of 85 existing hydropower reservoirs finds they collectively emit 1/6th the greenhouse gas emissions as currently assumed and "are not major contributors to the greenhouse gas problem."
Paper co-author Dr Johnathan Cole notes however that just 17% of potential hydropower reservoir sites are being used and "impacts vary based on reservoir age, size, and location."
Even though hydropower emits no greenhouse gas as it generates electricity, when a reservoir is constructed and new land flooded, for a time period greenhouse gases are emitted from the flooded land, at levels varying based upon what sort of habitat was flooded.
As paper lead author Nathan Barros concludes,
The bottom line is that per unit of energy, hydroelectric generation produces much less carbon dioxide and methane emissions than previous thought, but impact are not equal across all landscape. Reservoirs in tropical locations, such as the Amazon, emit more methane and carbon throughout their lifecycles. (Science Codex)
Of course, when it comes to large-scale hydropower though this research shows that the greenhouse gas concerns are not nearly as great as we thought, construction of dams and reservoirs of this size still have large impacts (often negative) on the ecosystem in which they are located, as well as having impacts on water flow downstream.