The Collaborative International Dictionary of English v.0.48:

Hydro-electric \Hy`dro-e*lec"tric\, a. [Hydro-, 1 + electric.]
   Pertaining to, employed in, or produced by, the evolution of
   electricity by means of a battery in which water or steam is
   used.
   [1913 Webster]

   Hydro-electric machine (Physics), an apparatus invented by
      Sir William Armstrong of England for generating
      electricity by the escape of high-pressure steam from a
      series of jets connected with a strong boiler, in which
      the steam is produced.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hydroelectric
    adj 1: of or relating to or used in the production of
           electricity by waterpower; "hydroelectric power"

Moby Thesaurus II by Grady Ward, 1.0:

21 Moby Thesaurus words for "hydroelectric":
   battery-powered, dynamoelectric, electric, electric-powered,
   electrified, electrifying, electrochemical, electrodynamic,
   electrokinetic, electromechanical, electrometric, electromotive,
   electropneumatic, electrostatic, electrothermal, galvanic,
   galvanometric, photoelectric, piezoelectric, static, voltaic