> So far it seems the best power source we have is hydroelectric. Of course it's only possible in some places. This can devastate certain species (eg salmon) but in terms of cost, risks, environmental impact and power output, hydro is hard to beat if it's an option.

Note that according to one recent study, reservoirs also contribute significantly to greenhouse gas emissions. Previous HN discussion [1].

It's just one study and its quantified result should probably be debated, but it's general argument that reservoirs contribute to greenhouse gas emissions sounds well-reasoned to me.

[1] https://news.ycombinator.com/item?id=12884030