Source : Webster's Revised Unabridged Dictionary (1913)

Wasteweir \Waste"weir`\, n.
   An overfall, or weir, for the escape, or overflow, of
   superfluous water from a canal, reservoir, pond, or the like.

Source : WordNet®

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