Source : Webster's Revised Unabridged Dictionary (1913)

Tailrace \Tail"race`\, n.
   1. See {Race}, n., 6.

   2. (Mining) The channel in which tailings, suspended in
      water, are conducted away.

Source : WordNet®

tailrace
     n : a watercourse that carries water away from a mill or water
         wheel or turbine