Query "node", "way" and "relation" elements with "generator:source" and "power:source" equal to "hydro" value:
(node["generator:source"="hydro"];
way["generator:source"="hydro"];
relation["generator:source"="hydro"];
node["plant:source"="hydro"];
way["plant:source"="hydro"];
relation["plant:source"="hydro"];)