WordNet (r) 3.0 (2006):

head gate
    n 1: regulator consisting of a valve or gate that controls the
         rate of water flow through a sluice [syn: sluicegate,
         sluice valve, floodgate, penstock, head gate,
         water gate]
    2: a gate upstream from a lock or canal that is used to control
       the flow of water at the upper end