Interesting idea. As jdietrich says, I presume the reason we don't see this is simply the question of scale and cost. If you want to catch kilotons of water, a

Interesting idea. As jdietrich says, I presume the reason we don't see this is simply the question of scale and cost. If you want to catch kilotons of water, a reservoir is far cheaper than an enormous raised platform. On the flipside, there's the question of how far down you can get the water to drop, before feeding it to your generator. This is something already taken advantage of - https://en.wikipedia.org/wiki/K%C3%B6lnbrein_Dam#Reisseck-Kreuzeck_power_plants https://en.wikipedia.org/wiki/K%C3%B6lnbrein_Dam#Reisseck-Kr...