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Guarding the Treasure Valley: A history of Lucky Peak Dam

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Guarding the Treasure Valley: A history of Lucky Peak Dam

By Hannah Mitchell, U.S Army Corps of Engineers

Lucky Peak Dam was the first dam to be designed and constructed by what was then the new Walla Walla District of the U.S. Army Corps of Engineers (USACE). Since its construction, Lucky Peak is estimated to have prevented over a billion dollars of flood damage.

Every year, snowmelt from the surrounding mountains flows into creeks and streams that join the Boise River. When flows reach 7,000 cubic feet per second (cfs) or higher, the river is considered at flood stage. Flooding records in the Treasure Valley date to about 1865, with some floods greatly exceeding the 7,000 cfs mark. The flood of 1896 was measured at 35,500 cfs and the 1943 flood reached 25,000 cfs.

For a long time, the growing community and state capitol of Boise, Idaho, had to contend with the sometimes violent nature of the Boise River, in a cycle of building and rebuilding. Arrowrock Dam, belonging to the U.S. Bureau of Reclamation, remained the only storage dam in the basin until the 1950s. The dam was designed as an irrigation project, constructed on the Boise River in 1915 and raised 5 feet in 1937.

Construction of Lucky Peak Dam, a rolled earthfill dam, began in 1950 and was completed in 1955.

But it was not enough, and something needed to be done for flood control.

In 1946, Lucky Peak Dam was authorized under the Flood Control Act. Lucky Peak was named after a gold mining camp about 3 miles north of the planned project footprint, just downstream of Arrowrock. Around the same time, USBR began construction on Anderson Ranch Dam, 42 miles upstream of Arrowrock Dam. This system of three dams would provide flood control and irrigation for the Treasure Valley.

While Lucky Peak was in the design phase, USACE began making organizational changes. The Treasure Valley had been under the jurisdiction of the USACE Portland District. However, the authorization of Lucky Peak in 1946, and the authorization of five dams just the year before, led USACE to survey towns in the Pacific Northwest with the goal of establishing a new district. Boise was surveyed, along with Pendleton, Tri-Cities, Spokane and Walla Walla. Ultimately, the Walla Walla District was established on Nov. 1, 1948.

Growing up together

When the Walla Walla District was established, the Lucky Peak project was still being designed. Several advance engineering studies had been funded for fiscal year 1948, but the Portland District was just sketching out the details when it passed Lucky Peak to the new district.

The Portland District passed several other projects to the new district, including the recently completed Mill Creek Dam and McNary Lock and Dam, which was in the middle of construction. Thus, Lucky Peak Dam became the first dam designed and constructed by the Walla Walla District.

Construction began in 1950 and was completed in 1955. Lucky Peak Dam was built as a rolled earthfill dam, 1,700 feet long at the crest, holding back Lucky Peak Lake. The lake is 12 miles long, with a storage capacity of 264,400 acre-feet of water. During flood seasons, this capacity is used to retain excess flows of the Boise River. In dry seasons, the reservoir water is released to maintain the river for local farmers and irrigators.

And Lucky Peak is not working alone. Anderson Ranch Dam was completed in 1950, becoming the second USBR dam on the Boise River. USACE and USBR can coordinate operations of these three storage dams, making effective use of a combined 1 million acre-feet of water storage capacity.

However, water supply is not the only consideration. Lucky Peak Lake bisects the Idaho Department of Fish and Game’s (IDFG) Boise River Wildlife Management Area, a major winter range for deer and elk. Development of the Lucky Peak project required addressing the impacts to wildlife, as well as the needs of the community surrounding the project.

The city of Boise intertwines urban and outdoor life. The people of the region enjoy fishing, hunting and general outdoor recreation. Yet, when construction of Lucky Peak was authorized, no commitments were made to develop recreation on project lands. When the project was completed, a boat launch ramp was built at the dam, with a play area and beach halfway up the reservoir. It soon became apparent that these facilities would not satisfy the recreation needs of the community. And Boise was still growing.

KEEP YOUR FORESTS GREEN on Lucky Peak Dam was an extension of the KEEP IDAHO GREEN campaign of the 1950s. The message was rephrased because Lucky Peak Dam serves the entire U.S., not just Idaho.

Today, there are 4,079 acres of USACE-managed lands used for recreation, wildlife habitat and operations purposes around the Lucky Peak project. USACE provides seven park areas, four boat launch ramps and 80 dispersed recreation sites accessible only by boat. IDFG manages Lucky Peak’s non-park lands under license to maintain the habitat for deer and elk.

There are also three Lucky Peak State Park units: Discovery Park, Sandy Point and Spring Shores Marina. These are operated by the Idaho Department of Parks and Recreation on the shores of Lucky Peak Lake. Ada County Parks and Waterways provides courtesy dock strings under license at USACE boat ramps and remote boat-in sites.

These recreation opportunities have developed along with Boise. The Lucky Peak project sees an average of nearly 1 million visits every year.

A show of power

During the dedication of Lucky Peak Dam on June 23, 1955, operation of the project was illustrated by opening the outlet gates and creating the first “rooster tail” display at Lucky Peak. The rooster tail uses two of the dam’s original six gates to release water from the reservoir back to the Boise River. Using a “flip bucket,” the spray is aimed high into the air to reduce the water’s scouring, erosive force before it falls back into the stream channel.

The rooster tail uses two of the dam’s original six gates to release water from the reservoir back to the Boise River. Using a “flip bucket,” the spray is aimed high into the air to reduce the water’s scouring, erosive force.

When Lucky Peak Dam was built, rooster tail displays occurred regularly because the outlet gates made up the only release structure of the dam. However, the power of the rooster tail display began to raise questions: Was this powerful supply of water being wasted? Could it be harnessed to provide additional benefits for the region, say in the form of electricity?

At the time of Lucky Peak’s construction, adding hydropower generators was not economically feasible. Then, in the 1970s, a nationwide push to develop hydropower led the Walla Walla District to perform a study allowing for construction of a power plant at Lucky Peak.

After much debate, the Federal Energy Regulatory Commission awarded a license to the Boise Project Board of Control to undertake the project. Work began in 1984 to construct the power plant and reroute water releases through three hydropower turbines. The powerhouse, owned and operated by four irrigation districts within the Boise Project Board of Control, began generating power in October 1988.

As part of the several-year-long effort, a separate release structure had to be constructed to manage water releases while the original facilities were modified to accommodate the powerplant. The new release structure, in conjunction with the powerhouse, handles the majority of water being released from Lucky Peak Lake. The rooster tail, once a common sight, now only happens as an occasional public event when river flows are great enough to require releases exceeding the capacity of the powerhouse. The last display took place in April 2017.

When flood waters rise

The beginning of 2017 brought record-breaking amounts of rain and snow. Snow runoff from January to July was the second highest in the region since 1900. In February, the Boise River overflowed its banks, sending emergency managers into a flood fight that lasted more than 100 days.

Walla Walla District hydrologists worked closely with USBR and irrigators to manage the Boise River reservoir system during the flood fight. In addition, the district provided about 550,000 sandbags to Idaho communities and constructed a 4-foot-tall temporary emergency levee to stabilize 4,300 feet of riverbank, protecting homes and critical infrastructure.

It was a long flood, but not a big flood. Lucky Peak Dam played a role in the flood fight, and efforts by USACE and its partners were successful in preventing large-scale flooding in Boise and the surrounding communities.

While they cannot prevent flooding, storage dams like Lucky Peak are designed to absorb excessive flows into their reservoirs, thereby reducing the impact of heavy precipitation. Since its construction, Lucky Peak is estimated to have prevented over $1 billion dollars of flood damage.

A dedication to safety

Over the years, Lucky Peak Dam has been well-maintained and remains in good physical condition. While the dam remains essentially unchanged, Boise, just 10 miles downstream, has changed a lot.

When Lucky Peak Dam was completed in 1955, the population of Boise, according to U.S. Census statistics, was about 30,000. Today, Boise boasts a population of over 235,000. The Boise metropolitan area — which includes Nampa, Caldwell and other towns that have blended into Boise’s borders — has a population upwards of 800,000.

This means that, while the likelihood of dam failure remains low, the consequences of a potential dam failure are much greater than they would have been 70 years ago.

The Walla Walla District performs routine inspections and evaluations of all its dams and is dedicated to maintaining public safety. Because of Boise’s growth, Lucky Peak Dam has a higher priority for funding to examine potential risks and identify risk reduction measures.

Providing public safety through flood control has been a mission of the Walla Walla District since its beginnings 75 years ago. It is a mission that will continue to drive operations at Lucky Peak, and other projects, into the future.

Editor’s Note: This article was originally published by the U.S. Army Corps of Engineers and is reprinted with permission.