I know that when we had a major draught in Tennessee a few years ago, generation at the hydro plants was extremely limited. We just didn’t want to move too much water downstream, due to the need to keep the river navigation channels open. Hydro power generation was probably 1/10 of what it normally would be that year.
In times of excess rain, we have to spill water, which the power generation people refer to as “throwing away watts” since spilling does not go through the generator but is basically a means of flood control where spill gates are opened to allow a large amount of water to flow.
++!! for the info