Source : Webster's Revised Unabridged Dictionary (1913)

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

Source : WordNet®

water gate
     n : 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}]