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 Places (2000):

Hydro, OK -- U.S. town in Oklahoma
   Population (2000):    1060
   Housing Units (2000): 466
   Land area (2000):     0.604889 sq. miles (1.566655 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    0.604889 sq. miles (1.566655 sq. km)
   FIPS code:            36700
   Located within:       Oklahoma (OK), FIPS 40
   Location:             35.548882 N, 98.577762 W
   ZIP Codes (1990):     73048
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Hydro, OK
    Hydro