The Collaborative International Dictionary of English v.0.48:

Wasteweir \Waste"weir`\, n.
   An overfall, or weir, for the escape, or overflow, of
   superfluous water from a canal, reservoir, pond, or the like.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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