Owned and operated by
Seattle City Light, this Hydro Project supplies more than one-third of Seattle City Light's power. Water stored behind its 340 foot high -- 32 foot thick arch dam forms a lake that stretches 17.5 miles upstream to the Box Canyon Dam and contains 40,000 acre feet of water. When completed in 1967 the project utilized four turbines, each producing up to 208,000 kilowatts of electricity. In 1985 two more turbines and generators were added, bringing its total output to 1,024 Megawatts.
This is an interesting project, first because an arch dam was employed instead of a gravity dam, saving gazillions of yards of concrete, and secondly because the powerhouse is nowhere to be seen. Unless, that is, one goes underground. It is within the mountain/hill adjacent to the dam, ensconsed within pure bedrock. The site is at a narrow canyon, chosen because it allowed construction of a dam of minimum possible length.
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