This is how they made the lake: https://en.wikipedia.org/wiki/Gatun_Dam https://en.wikipedia.org/wiki/Gatun_Dam I'm assuming flooding the other way around was

This is how they made the lake: https://en.wikipedia.org/wiki/Gatun_Dam https://en.wikipedia.org/wiki/Gatun_Dam I'm assuming flooding the other way around was more expensive than creating a fresh water reservoir by building a dam.