water gauge

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An instrument indicating the level of water, as in a boiler, tank, reservoir, or stream.

from the GNU version of the Collaborative International Dictionary of English.

• 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A glass column which displays water levels.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun gauge for indicating the level of water in e.g. a tank or boiler or reservoir