A river flows all the time, but people and businesses need more energy at certain times of day. Because of this, more water is allowed to flow through dams during high demand times. During the summer, high demand occurs in hot afternoons when more people run their air conditioners. At these times, much hydropower is generated. Late at night, there is less need for electricity and water is stored behind the dam reservoir to meet time of peak demand. In this way, a reservoir behaves like a constantly charging battery. During the day, it discharges faster than it can charge up. At night, energy is recharged as stored potential energy.
Sometimes a special reservoir is created for pumping stored water into a river or lake behind a dam. This can be useful to generate more electricity during times of high demand. The Seneca Pumped Storage Facility supports the Kinzua Dam in Pennsylvania. During high demand times, water is pumped from the Seneca Pumped Storage Facility to the Allegheny Reservoir. This helps provide power when people need it most.
