a passageway through which surplus water escapes from a reservoir, lake, or the like.

1885–90; spill1 + way1

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World English Dictionary
spillway (ˈspɪlˌweɪ)
wasteweir, Also called: spill a channel that carries away surplus water, as from a dam

Collins English Dictionary - Complete & Unabridged 10th Edition
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Encyclopedia Britannica


passage for surplus water over or around a dam when the reservoir itself is full. Spillways are particularly important safety features for earth dams, protecting the dam and its foundation from erosion. They may lead over the dam or a portion of it or along a channel around the dam or a conduit through it.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
During that period, the hydro power was turned down causing excess water to go
  over the spillway.
In the video, several people are cruising in a small motorboat below the
  spillway of a lock or a dam while fish fly all around.
Water that goes over a spillway as opposed to through a turbine picks up
  bubbles of nitrogen gas from the atmosphere.
The spillway is designed to provide a safe path for the excess flows.
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