The Collaborative International Dictionary of English v.0.48:

Hydro \Hy"dro\, n.
   1. A hydro-a["e]roplane. [obsolescent]
      [Webster 1913 Suppl.]

   2. hydroelectric power; -- also used attributively.
      [PJC] Hydro

The Collaborative International Dictionary of English v.0.48:

Hydro- \Hy"dro-\, Hydr- \Hy"dr-\
   1. A combining form from Gr. ?, ?, water (see Hydra).
      [1913 Webster]

   2. (Chem.) A combining form of hydrogen, indicating hydrogen
      as an ingredient, as hydrochloric; or a reduction product
      obtained by hydrogen, as hydroquinone.
      [1913 Webster]

U.S. Gazetteer (1990):

Hydro, OK (town, FIPS 36700)
  Location: 35.54882 N, 98.57862 W
  Population (1990): 977 (473 housing units)
  Area: 1.6 sq km (land), 0.0 sq km (water)
  Zip code(s): 73048