Definitions for water gauge
Random House Webster's College Dictionary
any device for indicating the height of water in a reservoir, boiler, etc.
Origin of water gauge:
water gauge, water gage, water glass(noun)
gauge for indicating the level of water in e.g. a tank or boiler or reservoir
A glass column which displays water levels.
a wall or bank to hold water back
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