That's exactly when pumped hydro shines. Increasing the storage reservoir capacity is cheap. It's the turbines and infrastructure that's expensive, but that doesn't change if you're only increasing the capacity.
All the cheap storage capacity with natural flow have been taken.
Pumped hydro doesn't need natural flow, because it uses pumps. All it needs is a height differential. Here's 22,000 in Australia alone. There are millions worldwide.