There are clearly no free lunches, and this holds for any source of energy.
One thing you didn't mention is the fact that the area above the dam is usually flooded (which may yield another ecological issue). On different grounds, building the dam may also be not very eco-friendly (a lot of concrete in there, that needs to be produced somehow).
This being said, hydro is very clean, but maybe not 100% clean (such a thing does not exist IMO).
Thank you very much for the additional info sir!
I agree with that, If not I'm wrong it's because of siltation. As time flies sediment deposits build up under the dam making the it shallower.
Yes there's no such thing as a perfect energy source. We have no choice but to pick the better ones and improve it along the way.
Again, many thanks to you guys in @steemstem and @curie! You're all awesome. XD