Arrowrock Dam is a concrete arch dam in the western United States, on the Boise River in southwestern Idaho, east of Boise. Opened 110 years ago in 1915, it is located on the border of Boise and Elmore counties, upstream of the Lucky Peak Dam and reservoir. The spillway elevation for Arrowrock is 3,219 feet above sea level and its primary purpose is to …Arrowrock Dam is a concrete arch dam in the western United States, on the Boise River in southwestern Idaho, east of Boise. Opened 110 years ago in 1915, it is located on the border of Boise and Elmore counties, upstream of the Lucky Peak Dam and reservoir. The spillway elevation for Arrowrock is 3,219 feet above sea level and its primary purpose is to provide irrigation water for agriculture.