Well I've never actually researched it, but I would assume a Wikipedia search would get you all the gory details.
But from being through there watching the water move and the sounds that accompany it, I assumed they use the natural hydrostatic pressure, letting the water in from up above and letting it out to down below.
When we were on the sandbar waiting to go up I noticed water rushing out before they open the gates and the level on that little sandbar just down from the falls went up about 4 inches.