From projects that were done in the forties, fifties, sixties, and seventies, there was more impact. Even in those cases the ecosystem actually adjusts and you might be changing from more river base to a lake base. You create a different kind of habitat, but if you consider the whole scheme of things, you're still in balance.
I think the most important consideration for large-scale hydro is the ability to store water and manage it. The fact of how it works compared to other renewables, such as wind and solar, which are non-dispatchable...nothing works. The best battery possible is large hydro.