WordNet (r) 3.0 (2006):

floodgate
    n 1: something that restrains a flood or outpouring; "suspension
         of surveillance opened the floodgates to illegal
         immigrants"
    2: 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:

46 Moby Thesaurus words for "floodgate":
   aboideau, air lock, avenue, blowhole, channel, chute, debouch,
   dock gate, door, egress, emunctory, escape, estuary, exhaust, exit,
   flood-hatch, flume, gate, head gate, lock, lock gate, loophole,
   opening, out, outcome, outfall, outgate, outgo, outlet, penstock,
   pore, port, sally port, sluice, sluice gate, spiracle, spout, tap,
   tide gate, vent, ventage, venthole, vomitory, water gate, way out,
   weir