The province of Québec stops or reduces the flow of many of its hydroelectric dams at night due to the much market lower prices spot prices for electricity, especially since nuke plants flood the markets with unused capacity.
That's great - but really the dams need to be able to help handle peak grid capacity, and to do that they need to be able to generate more than they were originally designed for so that they can trade off against those other sources that weren't around when the dams were built
There's a spare generator at the Manic 5 dam for super-high demand events like Christmas, Easter, and also cold fronts, and maybe a spare gas plant somewhere. Otherwise the purpose of holding back water at night is to prevent reservoir depletion in low-precipitation years, rather than building extra capacity for high-demand peaks.
