Hydroelectric sites generate electricity from falling water. 
Combinations of water turbines and generators (known as water turbine generators or hydro generators) can generate power when installed along the path of water flowing from an elevated dam, reservoir, or tank that may be used as a water supply or for irrigation. Electricity can be generated as long as a difference in elevation exists, making this method of power generation quite versatile.
Most commonly, electricity is generated by using the power of water to turn a water turbine. 

Electricity consumption varies significantly from season to season and even daily, from day to night. Providers manage these changes to provide stable, high-quality power by pumping and storing water at night to prepare for daytime peaks and employing the best mix of complementary power sources that may also include oil, gas, coal, or other thermal sources as well as nuclear.