spillway

n. channel which directs surplus water away from a reservoir or lake

A spillway is a structure used to provide for the controlled release of flood flows from a dam or levee into a downstream area, typically being the river that was dammed. Spillways release floods so that the water does not overtop and damage or even destroy the dam. Except during flood periods, water does not normally flow over a spillway. In contrast, an intake is a structure used to release water on a regular basis for water supply, hydroelectricity generation, etc. Floodgates and fuse plugs may be designed into spillways to regulate water flow and dam height. Other uses of the term "spillway" include bypasses of dams or outlets of a channels used during highwater, and outlet channels carved through natural dams such as moraines.
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Noun
1. a channel that carries excess water over or around a dam or other obstruction
(synonym) spill, wasteweir
(hypernym) conduit

(n.)
A sluiceway or passage for superfluous water in a reservoir, to prevent too great pressure on the dam.
  

A constructed passage for surplus water to run over or around a dam. (DOI3)