Right, I get that there are alternatives that would be used instead, what I was surprised at was characterizing "hydro only works in mountainous areas" as flatly false, if we take "works" to mean "is worth doing", which is a common usage and I think a fair reading of it, and since it seemed to me like there would be large differences in the economics of it, depending on local geography.

Low hills, where the hilltops are not otherwise in use, may suffice. Some places have very deep gullies or underground cavities that would serve in place of or augment hills.

The point is that whether a storage medium is practical in an area depends more on the landscape of choices than on the physical topography. Even Nebraska might have deep cavities that would serve in place of hills. But the extrema are extreme.