Construction History

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Arrowrock Dam helped provide growth to the Treasure Valley. The dam was constructed from 1911-1915 and was recorded to be the tallest dam in the world until 1925, when it was surpassed by a dam in Switzerland that was a few feet higher.

Arrowrock Dam was originally 350 feet tall with a crest length of 1,150 feet. It was repaired and raised 5 feet between 1935 and 1937, increasing its storage by 9,000 acre-feet with a total storage capacity of 272,200 acre-feet.

The original construction involved the use of a rather high proportion of sand cement, and by 1935, the concrete on the downstream face of the structure showed deterioration due to climatic conditions. The downstream face of the dam and the spillway channel surfaces were repaired as a result.

Two rows of 10 Ensign valves were originally installed on the upstream face of Arrowrock Dam at the entrance to conduits used to pass water to the downstream river. The lower row of valves were removed and replaced with clamshell gates between 2001 and 2004. The other row of Ensign valves remain in place and are rarely used. With the new clamshell gates, maintenance and inspections can be done without lowering the reservoir, whereas with the Ensign valves, a reservoir drawdown was required.

The Boise Project Board of Control completed construction of a 15 MW powerplant at Arrowrock Dam in March 2010. The powerplant penstocks tie into two of the existing 66" outlet conduits. The project also updated the transmission line to the dam.

Related Links
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Boise & Arrowrock Railroad History

Last Updated: 1/19/16