The Collaborative International Dictionary of English v.0.48:

Watercourse \Wa"ter*course`\, n. (Shipbuilding)
   One of the holes in floor or other plates to permit water to
   flow through.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

watercourse
    n 1: natural or artificial channel through which water flows
    2: a natural body of running water flowing on or under the earth
       [syn: stream, watercourse]
    3: a conduit through which water flows [syn: watercourse,
       waterway]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "watercourse":
   adolescent stream, aqueduct, arroyo, beck, bed, bourn,
   braided stream, branch, brook, brooklet, burn, canal, channel,
   conduit, course, creek, creek bed, crick, culvert, donga, dry bed,
   duct, flowing stream, flume, fluviation, fresh, freshet, gill,
   gulch, gully, gullyhole, headrace, irrigation ditch, kill,
   lazy stream, meandering stream, midchannel, midstream, millstream,
   moving road, navigable river, nullah, race, racing stream, river,
   river bed, riverway, rivulet, run, rundle, runlet, runnel, sike,
   sluice, spill stream, spillbox, spillway, stream, stream action,
   stream bed, streamlet, streamway, subterranean river, swash,
   swash channel, tailrace, wadi, water carrier, water channel,
   water furrow, water gap, water gate, waterway, waterworks