How do hydro-electricity energy sources work?

Hydro electricity - power from flooded valleys is used to start a generator. A big dam is built and rainwater is caught and allowed through turbines, there's no pollution. It destroys the environment and habitats, rotting vegetation releases methane and carbon dioxide and the reservoirs can look unsightly when they dry. But, it can provide an immediate response, and there's no problem with availability unless there's a drought. The costs are high but there's no fuels so there are low running costs. Its good for generating in remote areas at a small scale. It is renewable.