What potential harm to the environment does hydroelectric energy cause?

May harm fish populations
Change temperature

Force relocation.
Fish prevented from swimming upstream

http://www.energy4me.org/energy-facts/environmental-protection/environmental-impact-by-source/

http://cdn.spectator.co.uk/wp-content/uploads/2013/01/Hydro-Fish.jpg

http://thewe.cc/thewei/_/images_4/china/two_people_look_at_three_gorges.j
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