Property Type: Institutional
Neighborhood: The Boise River Project  |  County: Ada  |  Building Status: Public  |  Year Built: 1915  |  Architectural Style: Arch Dam
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As the dirt turned to pavement on Arrow-rock Road, we were more worried about rain than our friend’s driving. We expected this. Back roads in Idaho usually aren’t paved. Soon after we started kicking up dust, we noticed that the water flow was different; moving, swirling. Nothing you would normally see on a calm reservoir behind Lucky Peak Dam. As we rounded the corner of the first dirt turn, we were greeted with a massive concrete monstrosity, discolored after years of weathering. The Arrow-rock dam was very big, and a great example of an arch dam. With cameras in hand, we were ready to take this beast head on.

Completed in 1914 and dedicated in 1915, Arrow-rock set a new world record for the highest dam ever built. At 350 feet tall, the record was not broken until the completion of the Hoover Dam in 1936. Work on the Arrow-rock dam began in 1910, when the Bureau of Reclamation decided that the Treasure Valley needed more irrigation for its growing farming communities, and control the flooding of the Boise River.

To get to the site with supplies and electricity was a feat in itself. The Bureau installed a turbine into Diversion Dam that generated electricity for working sawmills and concrete mixers at the construction site. While the power line isn’t still in use, you can hike the route and find interesting artifacts relevant to electrical history.

The destruction of mountain life to make way for the road that winds its way up the canyon was equally challenging. At many spots the workers blasted their way through the sheer mountain face to get to the dam building site. They didn’t carve through the rugged terrain just because they wanted to. It was for a railroad. The beginning platform of the railroad was at Barber Park, although the park wasn’t named so in 1910. The track ran all the way up to the dam site where crews unloaded supplies needed in construction.

When construction started early in 1912, over 1,400 workers flocked to the camp that provided food, shelter, a hotel, and even a YMCA for them and their families. Work proved to be fairly dangerous, as 12 people died and many more suffered serious injuries.

On the face of the dam you are able to see a row of ten rust covered holes. On days when the reservoir is high, the holes are continuously spewing massive amounts of water. These mystery holes are known as ensign valves. From 1915 to 2003, the valves controlled the flow of water out the dam. These aided the massive spillway at the side of the dam to prevent a sudden overflow of water by giving the water a route away from the dam that isn’t simply straight over.

Originally in two rows of twenty, the back ten were removed. The front ten, if removed, would compromise the structure of the dam. These are still in use. After being decommissioned, one of the ensign valves was set aside for public viewing. It is roughly 12 feet in diameter and very heavy and rusty.

The dam’s purpose was to irrigate the valley, as well as the serious flood controlling agent in the Boise area. It provided a large amount of people employment, in farming and construction. It also changed the layout of the Boise area, for example Front Street used to be next to the high river mark in the spring, but Arrow rock’s flood control abilities limited the reach of the river. This led to new areas of development in the downtown area. This taming of the river allowed for the expansion of the Treasure Valley in population and significant architecture.

One could argue that a large part of the BAP would not exist without Arrow-rock Dam. In 2008, an epic journey to install power generators ensued. The plan was successful, and Arrow-rock now helps provide electricity for Boise and the surrounding area.

Unfortunately, the Bureau of Reclamation was not into the trend of conservation of wildlife. Not a single fish has made it past the dam since 1915. This project taught us a lot about the importance of dams to not only the Boise area but the entire western United States as well.