The Collaborative International Dictionary of English v.0.48:

Spillway \Spill"way`\, n.
   A sluiceway or passage for superfluous water in a reservoir,
   to prevent too great pressure on the dam.
   [1913 Webster]

WordNet (r) 3.0 (2006):

spillway
    n 1: a channel that carries excess water over or around a dam or
         other obstruction [syn: spillway, spill, wasteweir]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "spillway":
   aqueduct, arroyo, bed, canal, creek bed, culvert, donga, dry bed,
   flume, gulch, gully, gullyhole, headrace, irrigation ditch, nullah,
   race, river bed, riverway, runnel, sluice, spillbox, stream bed,
   streamway, swash, swash channel, tailrace, wadi, water carrier,
   water channel, water furrow, water gap, water gate, watercourse,
   waterway, waterworks