Arrowrock Dam

Arrowrock Dam
Spillway and dam in May 1993
Location in southwestern Idaho
Country	United States
Location	Boise and Elmore counties, Idaho
Coordinates	43°35′44″N 115°55′20″W / 43.59556°N 115.92222°W / 43.59556; -115.92222Coordinates: 43°35′44″N 115°55′20″W / 43.59556°N 115.92222°W / 43.59556; -115.92222
Purpose	Irrigation
Construction began	1911 (1911)
Opening date	1915 (1915)
Owner(s)	U.S. Bureau of Reclamation
Dam and spillways
Type of dam	Arch dam
Impounds	Boise River
Height	350 feet (110 m)
Length	1,150 feet (351 m)
Reservoir
Creates	Arrowrock Reservoir
Total capacity	300,850 acre feet (0.371 km3)
Catchment area	2,200 square miles (5,700 km2)
Surface area	3,100 acres (12.5 km2)
Normal elevation	3,210 ft (978 m) AMSL

Arrowrock Dam is a concrete arch dam on the Boise River, in the U.S. state of Idaho. It opened in 1915 and is located on the border between Boise County and Elmore County, upstream of the Lucky Peak Dam and reservoir. The spillway elevation for Arrowrock is 3,219 feet (981 m) above sea level and its primary purpose is to provide irrigation water for agriculture.

In 1910 the Reclamation Service began to consider another storage facility further east on the Boise River. After several surveys, engineers decided upon the Arrowrock site which had previously been the site of a private irrigation venture under the direction of Arthur De Wint Foote yet failed for lack of funding. The Arrowrock site is at the confluence of the main channel and the south fork. This was to be the most ambitious project to date for Reclamation. At 348 feet (106 m), Arrowrock would be the largest concrete arch dam in the world. Prior to construction considerable preparatory work would need to be completed. As the structure was some 20 miles (32 km) up the Boise River from the Boise River Diversion Dam, routing supplies to the worksite would be a massive undertaking unto itself. The Reclamation Service elected to construct a new rail line on the old wagon road leading north to Idaho City. The railroad would begin at the Barberton mill near the Diversion Dam and extend to through a winding canyon up to Arrowrock. Even before the dam had been approved, Reclamation began work on the rail line.

Some significant problems existed with construction of the railroad. The Barberton Lumber Company owned the roads right-of-way. This meant the Reclamation Service needed to come to an agreement over ownership of the rail line. In an unprecedented move, the government agreed to lease the track from Barberton but run the actual locomotive. Part of this agreement stipulated that the line would remain a common carrier. This made the Arrowrock & Boise Railroad the first publicly owned line in the nation. The Service hid this fact from President Howard Taft when they applied for the Arrowrock dam’s approval. Fortunately for Reclamation, Taft failed to recognize the loophole and in June 1910, entire project went forward. Yet when the Oregon Shortline refused to honor the pact between Barberton and Reclamation, the Arrowrock & Boise terminal was reduced to a field just outside the Barber lumberyard. On August 22, 1910, the entire deal was finalized and work began on the line to the Arrowrock site.