Hydroelectricity is electricity that is made by the movement of water. It is usually made with dams that block a river to make a reservoir or collect water that is pumped there. When the water is "let go", the huge pressure behind the dam forces the water down pipes that lead to a turbine. This causes the turbine to turn, which turns a generator which makes electricity. This use of renewable energy produces less pollution than steam engines do. Some places such as Norway and Quebec get most of their electricity this way.
The way the electricity is produced does not harm the environment as much as fossil fuels like oil or coal do. Hydroelectricity is very powerful and safe, and produces no waste.
Hydroelectricity can be made very quickly. This makes it useful for times when demand for electricity is high. Water that has been stored in a dam can be released (let go) when needed, so the energy can be made quickly. This controllability also makes hydroelectricity a good match for less controllable intermittent energy sources. When the wind is not blowing or the sun is not shining, hydroelectricity can be released.

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