head gate

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

• noun A control gate upstream of a lock or canal.
• noun A floodgate that controls the flow of water in a ditch, sluice, race, or channel.

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

• noun regulator consisting of a valve or gate that controls the rate of water flow through a sluice
• noun a gate upstream from a lock or canal that is used to control the flow of water at the upper end