The Collaborative International Dictionary of English v.0.48:

Water gage \Wa"ter gage`\
   See Water gauge.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Water gauge \Wa"ter gauge`\ [Written also water gage.]
   [1913 Webster]
   1. A wall or bank to hold water back. --Craig.
      [1913 Webster]

   2. An instrument for measuring or ascertaining the depth or
      quantity of water, or for indicating the height of its
      surface, as in the boiler of a steam engine. See Gauge.
      [1913 Webster]

WordNet (r) 3.0 (2006):

water gage
    n 1: gauge for indicating the level of water in e.g. a tank or
         boiler or reservoir [syn: water gauge, water gage,
         water glass]