A hydro lake is exactly like a battery - potential energy filled by a river.

Also if Germany sends excess wind/solar electricity to Norway, and the hydro lake is emptied slower because of that, then the hydro acts exactly the same as battery storage — no pumping required. However some lakes have a minimum rate of outflow: when rivers need a minimum flow for sports, ecological, or downstream dam capacity reasons. Also during dry periods when lake capacity drops, the ability to be used as a battery also drops. https://news.ycombinator.com/item?id=28898426