Hydro power is not green. The benefit are it's cheap (in terms of cost to produce) and produces no CO2 emissions. The downside is it makes it harder to fish to travel from the ocean to their spawning sites. Like all power sources, it has benefits and drawbacks and it absolutely impacts the environment.
Hydro on the north of Quebec (most of % electricity produced) is not that a fish-reach area at all. But yes, on some areas in the world that's a concern.