For those who keep asking why we can't store water during times of low electrical demand and use it during high, take a look at how the
Lewiston Pump-Generating Plant at Niagra Falls does exactly that. Generates massive amounts of electricity during the peak load times. Really an awesome bit of engineering.
At night, a substantial fraction (600,000 US gallons (2,300 m3) per second) of the water in the Niagara River is diverted to the forebay by two 700-foot (210 m) tunnels. Electricity generated in the Moses plant is used to power the pumps to push water into the upper reservoir behind the Lewiston Dam. The water is pumped at night because the demand for electricity is much lower than during the day. In addition to the lower demand for electricity at night, less water can be diverted from the river during the day because of the desire to preserve the appearance of the falls. During the following day, when electrical demand is high, water is released from the upper reservoir through the pump-generators in the Lewiston Dam. The water then flows into the forebay, where it falls through the turbines of the Moses plant. Some would say that the water is "used twice".