Many hydroelectric plants with a reservoir are already energy storage systems. They can adjust their power output up and down. They can, for short periods, drain the reservoir faster than it is being filled. The seasonal patterns of rainfall, and energy demand, influence how much power a plant can output in terms of peak and average, both short and long-term.
At Niagara Falls, the falls can be diverted through the plants, nearly shutting the falls off. It isn't normally done, for both ecological and sightseeing reasons, but when power was needed after the 2003 blackout, the falls were reduced to about 15% flow: https://www.theglobeandmail.com/news/national/how-the-power-...
At Niagara Falls, the falls can be diverted through the plants, nearly shutting the falls off. It isn't normally done, for both ecological and sightseeing reasons, but when power was needed after the 2003 blackout, the falls were reduced to about 15% flow: https://www.theglobeandmail.com/news/national/how-the-power-...