It depends on where the dams are. I live in the North Eastern USA. Most of this part of the country (and eastern Canada) are powered by hydro. Niagara Falls doesn't turn off. Neither do dams, really. Even in the winter (-30F) and during summer droughts, they have water passing over the turbines. None of the dams in my area have needed maintenance in the time I've been alive (about 30 years).

It's cleaner and more reliable than any other kind of energy production on the planet including coal and oil. Nothing is 100% reliable, but hydro is the closest we're likely to get.