About a quarter of it is from Niagara Falls, ie: run of the river.
The amount that they divert varies by time: they want to keep the falls nice during the day, but divert more at night.
And a lot of dams are dual use: controlling water levels while generating electricity, so it really depends on weather. When it’s dry they can maybe do this, but when it’s wet, they gotta be careful. Or vice versa depending on which side it’s raining more on.
