How does hydroelectric energy create electricity?

Water released from the reservoir flows through a turbine, spinning it, which in turn activates a generator.

http://ffden-2.phys.uaf.edu/212_fall2009.web/bob_torgerson/dam.gif

http://www.renewableenergyworld.com/rea/tech/hydropower

http://www.sas.usace.army.mil/Portals/61/siteimages/Lakes/thurmond
/gates%20open.jpg