It means that my 'cleanish' verdict on hydro is severly questioned by the discovery that hydro can often produce quite a bit of methane which is considerably more potent as a warming gas than CO2 [x21]. This is "because large amounts of carbon tied up in trees and other plants are released when the reservoir is initially flooded and the plants rot. Then after this first pulse of decay, plant matter settling on the reservoir's bottom decomposes without oxygen, resulting in a build-up of dissolved methane. This is released into the atmosphere when water passes through the dam's turbines."
I guess it wouldn't be a problem if the biomass merely turned to CO2 because it is not fossil CO2, the problem is that it ends up as methane.
No one mentioned that in the Guardian article. Perhaps the fact that the Congo scheme isn't about damming mitigates the impact since it is a scheme to divert some of the flow down generation channels and then the outfolw is fed back into the river. Presumably that would mean considerably less die-back of vegetation under water.
New Scientist Breaking News - Hydroelectric power's dirty secret revealed
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