>1st, due to methane releases of decaying plant matter in the flood zones depending on what is flooded Hydro can actually produce over double the emissions of the worst coal plants, annually. For example the Curuá-Una dam in Pará, Brazil, has produced three-and-a-half times more emissions than an equivalent generating plant running on oil.

Thanks for the info, I had no idea. Is this a long term effect where reservoirs will continue to emit CO2 into the long term future, or is it just a one-off cost to transforming land into a lake? If it is just a temporary source of CO2, how long does it take for the emissions to peter out and at what point will the hydroelectric dam become even with coal in terms of emissions?