[jefftk]

If you just have water at the bottom of the hill you'll need to build a giant tank on the top of the hill. The best spots for pumped hydro have a natural depression elevated above a water source, so you can use that instead.

(I agree that this is different from traditional hydro locations, and so there are many good spots still available)

[bryanlarsen]

The best spots already have a reservoir up top, or most of one. Pumped hydro is so much cheaper than other long term storage options you can spend a little extra on the reservoir and still come out ahead of competitors.

50 year old example: https://en.wikipedia.org/wiki/Ludington_Pumped_Storage_Power...