Yes, that's what that is called, I'm just trying to figure out better ways to create them on steeper hillsides where you can not just build an earth wall. The traditional way used to be granite, but it's heavy work. I am just dreaming up a solarpunk future where we could plant reservoirs. I'm too lazy to carry stones. And the current method of building cheap, quick water reservoirs involves corrugated sheet metal, and that is ugly and awful to work with, and usually gets imported from elsewhere (as do bricks and concrete). My tree reservoir has the problem that the quick changes in water level of the reservoir could be bad for the trees.