GC: n

CT: An overflow spillway is most often constructed as part of a gravity dam or a buttress dam. The spillway section is lower than the other sections of the dam allowing water to flow over its top and down its front face.
Sometimes, the whole of the dam acts as an overflow spillway.
A close up of the spillway when it is dry shows that the top and downstream face are curved to control the flow of water over it.

S: http://britishdams.org/about_dams/overflow.htm (last access: 31 December 2014)

N: 1. 1889, from spill (1845, originally “a throw or fall from a horse,” from spill (v.); meaning “the spilling of a liquid, amount of spilled stuff” is from 1848) + way (Old English weg “road, path; course of travel; room, space, freedom of movement;” also, figuratively, “course of life” especially, in plural, “habits of life” as regards moral, ethical, or spiritual choices, from Proto-Germanic wegaz from PIE wegh- “to move”).
2. Water will probably never go over the top of the dam due to the spillways. The spillways work just like the overflow hole in your bathtub or sink at home. If the water ever gets up that high, it will go in the hole and down the drain, not over the top and onto the bathroom floor. The spillways are located 27 feet below the top of the dam, one on each side of the dam. Any water getting up that high will go into the spillways then into tunnels 50 feet in diameter, and 600 feet long which are inclined at a steep angle and connect to two of the original diversion tunnels. Each spillway can handle 200,000 cubic feet per second (cfs) of water. The flow at Niagara Falls is about 200,000 cfs, so there is the potential for two Niagara Falls here.

S: 1. OED – http://www.etymonline.com/index.php?allowed_in_frame=0&search=spillway&searchmode=none (last access: 31 December 2014). 2. http://www.usbr.gov/lc/hooverdam/History/essays/spillways.html (last access: 31 December 2014).

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CR: abutment, dam, hydroelectric power, hydroelectric power plant, reservoir , riprap.