It's true that hydro has a huge impact on the environment, but it is good for the climate. I think it can be worth it to build some hydro power to replace coal. If we don't, and the climate is fucked, then not having built the dam isn't helping the environment either.

If you pre-destory the environment you are trying to save I'm not sure what you've accomplished. If climate change is envirionmental problem 1a) then habitat destruction is certainly 1b) and low-density renewable energy sources are a huge problem here as are riparian-destorying damns.

The environment we're trying to save is large swaths of arable zones for human habitation. Fish are nice, but we need to be able to grow wheat.