Niagra falls doesn't run at full capacity because it takes away from the attraction of the falls themselves, and tourism is important there. They turn up capacity after hours, and the falls slow down.

Not only that, they use the gravitational potential of the falls to store massive amounts of energy when there's a surplus. Way cheaper to hold or even pump the water back up to the reservoir at the top than build lithium batteries. So yeah, as a local, can confirm they turn Niagara Falls (partially) off at night. Thanks to the Falls and several nuclear plants on Lake Ontario, Upstate NY and Southern Ontario have some of the lowest carbon electricity in the countries. Quebec is even better with basically all of their power coming from hydro.

See also https://en.wikipedia.org/wiki/International_Control_Dam