The tail race, containing tail water, is a channel that carries water away from a hydroelectric plant or water wheel. The water in this channel has already been used to rotate turbine blades or the water wheel itself. This water has served its purpose, and leaves the power generation unit or water wheel area.
In hydroelectric dams, the tail race is at a much lower level than the height of the reservoir behind the dam. This difference in height corresponds to the amount of hydropower that can be obtained from the water, and the height difference is known as the hydraulic head. This change in height corresponds to a change in gravitational potential energy. Some of the gravitational potential energy from the water above the dam was used to spin the turbines and generate electricity. Water flowing from a hydroelectric plant in the tail race eventually joins the natural flow of water.