Hydropower is energy obtained from flowing water. The most common source of hydropower is the dam however, it can be directly used as a mechanical force or a heat source or heat sink. In some regions (such as eastern Canada) "hydro" is a synonym for "electricity." Look for "electricity" in other categories.
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Information on renewable energy itself, but not primarily relating to water energy sources, belongs in the main category on Science/ Technology/ Energy/ Renewable/.