The Robert Moses Niagara Power Project runs at a deficit of water during the day once the morning really picks up and through peak hours into the evening. On a slightly longer timescale it also runs at an overall deficit during the workweek, drawing down water throughout the week until electricity use drops on the weekend. At that point it relies significantly less on the reservoir (and more on the forebay) and fills back up.
There are also 13 turbine generating units in RMNPP, while Lewiston pumped storage (the reservoir) has 12 much smaller (~1/10th the size) turbines as well. This allows quite a bit of flexibility in terms of output from what is a much larger power plant overall.
EDIT: Oh, and the reservoir was built on Native American land, which resulted in a Supreme Court case related to the Federal Power Act (I believe since it was treaty-breaking, can't quite recall) in case anyone is interested in that sort of thing.