The ecological impact of dams is totally local, but the ecological impact of CO2 is global. If we were to power the entire earth with hydro, the local damage to ecosystems would be a very small price to pay indeed.
Dams are obviously better than fossil, but I think a case can be made that fusion would be better than dams.
Also, using a fossil plant is harmful, using a dam is not. The damage was done when it was built.
I've seen arguments that some dams where the water level varies a lot produce a lot of GHG's due to decaying plant matter.
But yes, in general compared to fossils it certainly wins.
as opposed to figuratively terrible?
A lot of the impact can be mitigated by building fish pass channels, building new habitat such as compensation channels, and controlling the rate of change of water levels.
Any specific good material that I could read more on this?