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]

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "water gate":
   aboideau, air lock, aqueduct, arroyo, bed, canal, creek bed,
   culvert, dock gate, donga, dry bed, flood-hatch, floodgate, flume,
   gate, gulch, gully, gullyhole, head gate, headrace,
   irrigation ditch, lock, lock gate, nullah, penstock, race,
   river bed, riverway, runnel, sluice, sluice gate, spillbox,
   spillway, stream bed, streamway, swash, swash channel, tailrace,
   tide gate, wadi, water carrier, water channel, water furrow,
   water gap, watercourse, waterway, waterworks, weir