We tend to think of hydro-electric power as a totally clean form of energy, but there is a carbon footprint here too. According to figures obtained by Vincent St. Louis, who is a scientist at the Canadian University of Alberta, man-made reservoirs (of which one quarter are used for hydrolectric power production) release one billion tonnes of CO2 annually on top of another 70 million tonnes of methane, which has a global warming capacity of around 100x that of CO2. The problem is that bacteria break down plant materials submerged under their huge reservoirs into greenhouses gases. 80% of electricity is provided by Hydropower in Brazil, as an example of a tropical country, and Norway produces almost 100% of its electricity from hydroelectric sources. Canada and Switzerland, too, use the technology on a large scale since both are well provided for by rivers.
[The above figure that methane has 100x the global warming potential of CO2 might be disputed since it is often cited that it is nearer 20x. However, that is the average taken over 100 years, if equal volumes of methane and CO2 were emitted into the atmosphere, allowing that methane is oxidised in the troposphere to CO2 over a period of about 12 years. It is the "instantaneous radiative warming factor" that matters, since this would account for the relative effects of a steady release of the two gases, rather than a one-off emission].
The UN has decided to evaluate whether some countries might actually be better off by constructing coal and gas-fired power stations, which is an especially touchy point when heavy financial investments have been pledged, and certainly for the developing nations, e.g. Brazil. According to St.Louis, over a period of 100 years, hydro-dams will account for 7% of the global warming from all human activities. A typical example is the 250 MW Balbina dam in Brazil, which was created by flooding 2,500 square kilometers of Amazonian rainforest, the emissions from which are reckoned at 25% - 38% higher than from a coal-fired power station of equivalent capacity.
The degree of the emissions depend on the area and the depth of the reservoir, and on the nature of the underlying vegetation, and the deeper the better it would seem. However, each dam needs to be evaluated on its individual basis. Philip Fearnside, a conservation biologist at the National Institute for Amazon Research in Manaus, has concluded that the problem of the Balbina and Tucurui dams (the latter with 20x the generating capacity, but a dam area some 300 km^2 less than Balbina) is worse than previously thought, and that an average tropical hydropower plant emits four times as much carbon during the first four years of its life than a comparable fossil-fuel fired power station. Others have argued that the large emissions are caused by poor system design, and could be improved.
The jury is out still, since "The big issue is what would have happened if the reservoir hadn't been there," so sums-up Mike Acreman, a professor at the UK Centre for Ecology and Hydrology in Wallingford, Oxfordshire. "You can't go to oine and measure the methane coming off the surface and say that that was definitely caused by the hydropower scheme." Nonetheless, he does concede, "Perhaps hydropower is not as green as we thought. A lot of these tropical hydropower schemes would have been made by simply flooding a forest. There would have been a lot of trees and plants, and you need to think about what happens to all that carbon."
To my mind there are two issues here. Global warming and the decline in world oil and gas reserves. In the UK we make most of our electricity from gas, along with coal and nuclear, but hydropower is renewable unlike these finite and rapidly dwindling resources. I think we will need as much hydropower as possible, since the depletion of fuel resources will most likely decimate human civilization ahead of climate change.
"Hidden dangers," by David Adam, The Guardian: http://environment.guardian.co.uk/