Re: Energy "storage" via existing hydro
One problem is maintaining flow in a river or you wind up with big tides downstream of the dams. Hydro plants can throttle a little bit, but it's better to think of them as base load plants.
There are pumped storage facilities in different parts of the world that aren't on a river and do what you are describing. They take excess power from the grid to pump water uphill and release it when needed. The one in Wales is about 25% efficient. That's not huge, but much of the power they use is from wind and the turbines would have to be shut down if the pumps weren't using it so that 25% figure has to be adjusted from a purely kWh calculation. I'd like to see more of that type of facility being built.