Hydro means water. Hydro power is using the energy in falling water to make electricity. You find hydro power close to mountains because the snow melts and rain falls and forms streams and rivers. They build dams to store water for dry seasons. They put the power plant below the dam to catch the falling water they use to generate electricity.
This is the order in which hydro power is changed into electricity.
1. Water flows down through the dam and turns the turbine.
2. The turbine turns a shaft which turns magnets past coils of copper wire. This is called a generator.
3. This generates electricity which is sent out over wires to houses, schools and businesses.