It takes a lot of water to hold a little energy.

A cubic meter of water (1000 liters) weighs a metric ton, that is, 1000 kg.

If that fell 100 meters (a little more than 300 feet) that is

   10 m/s^2 * 1000 kg * 100 meter

or 10^6 J. That would generate 1kw of electricity for 1000 seconds. (Enough to bake 5 potatoes in the Microwave.)

That's a substantial amount of energy but it is a huge amount of water and it is a equivalent to a pressure of 10 atmospheres which would have blown up your plumbing!