[Schroedingersat]

Yeah this is a thing. If you have enough hydro you can just turn it off entirely when the renewables are going and have a 50/50 mix.

If not, it's called pumped hydro storage (or most precisely it's blie field on river pumped hydro if it's already dammed and on a river). You need to trap the water somewhere so you don't have to pump it too far. This involves building a lower reservoir and adding pumps or modifying the turbines to be two way.