The problem with that is the amount of energy stored in the water is proportional to the pressure as well, you'll see very few low head hydro electric dams because they're not very efficient, the pressures involved in real hydro plants are massive. I know there has been some testing done on low-head dams for daming something small rivers, but they're not particularly practical (just think of the building permits alone) for a hobbyist.
It's a good idea for home water re-use though, the biggest problem being sanitation.