It a lot of cases that doesn't happen. The water levels in the headwaters and the amount of flow on the downstream river are controlled with goals other than p

It a lot of cases that doesn't happen. The water levels in the headwaters and the amount of flow on the downstream river are controlled with goals other than power production. Like not causing floods, and having enough water depth for navigation. In Ontario, Canada, if the plant doesn't run at capacity, generally the water is sent through the spillway without generating power. I'd assume that to be common in many hydro-electric installations.