The Collaborative International Dictionary of English v.0.48:

Water gate \Wa"ter gate`\
   A gate, or valve, by which a flow of water is permitted,
   prevented, or regulated.
   [1913 Webster]

WordNet (r) 3.0 (2006):

water 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]