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]