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 A
shaft spillway
is sometimes called a "morning glory" spillway.
It is a hollow tower or shaft, usually circular, which has
a funnel at its top. The tower sits in the reservoir
near the dam.
 When
the reservoir level rises above the top of the spillway, the
water flows over its sides and down into the shaft. At the
bottom of the shaft, it enters a tunnel. The tunnel usually
runs under the dam, and diverts the water to join the river
downstream.
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