Photo By Hannah Mitchell | Dworshak Dam changed many times during its design. The original plans in 1948...... read more read more
In May 1948, floodwaters on the Columbia River overtopped the cofferdam where construction was underway on McNary Lock and Dam. This flood, one of the largest on record, was one of many documented in the region since the mid-1880s. The propensity for flooding in the Northwest sparked much discussion about regional flood control.
Through the 1950s, multiple reports marked the Clearwater River as an ideal location to build a large storage dam that could provide necessary flood control. Different plans were proposed, some for one dam, some for two. Multiple construction sites were suggested, but the conversation landed again and again on Bruces Eddy.
Bruces Eddy was a circular current of water, the result of a sharp bend in the North Fork of the Clearwater River. The eddy was named after engineer Bruce Lipscom, who lost his life there at the turn of the 20th century. Downstream of the eddy, the Clearwater River cut through a narrow gap in steep granite gneiss cliffs. It was here that Bruces Eddy Dam would be built.
When the project was authorized by the Flood Control Act of 1962, Bruces Eddy Dam was renamed Dworshak Dam, in honor of the late Senator of Idaho, Henry C. Dworshak.
Growth spurt
Dworshak Dam changed many times during its design. The original plans in 1948 described that it would be 370 feet tall. Later plans increased the height to 570 feet, then 630 feet.
When Dworshak Dam was constructed, its final height was 717 feet tall.
Dworshak’s size was almost unprecedented. As the design grew taller and taller, there was serious discussion about its shape and structure. What type of dam would be best? Should it be arched or straight-axis? Should it be rockfill or made of concrete?
An arch dam is curved upstream, allowing the weight of the reservoir to press mainly against the walls of the river valley, not the dam itself. Meanwhile, a dam built with a straight axis, also known as a gravity dam, relies on its own weight and the force of gravity to keep it stable.
Hoover Dam, dedicated in 1935 and operated by the US Bureau of Reclamation, is 726 feet tall, comparable to Dworshak. It was built as a concrete arch-gravity dam, meaning it benefits from the physics of the arch while also having the strength and thickness of a gravity dam.
Dworshak Dam was initially planned as an arch dam. However, the type of rock in the Bruce’s Eddy area was determined to be unsuitable for an arch dam, so engineers designed Dworshak with a straight axis.
Other plans dictated the project as a rockfill dam, however, there was no precedent for a rockfill dam as tall as Dworshak. With such a large structure, safety was the most important consideration when finalizing the design. Concrete was the safer choice, so concrete was the final choice.
And so, Dworshak Dam was built as a concrete gravity dam. This design was best suited for the construction site and maximized the safety of everyone downstream.
Today, Dworshak Dam is the third tallest dam in the United States and the tallest straight-axis concrete dam in the Western Hemisphere.
The tallest straight-axis concrete dam in the world is the Grande Dixence Dam in Switzerland. At 935 feet tall, it is 218 feet taller than Dworshak.
Pouring the concrete
In Dworshak’s construction, the first bucketful of concrete was poured on June 22, 1968. To say this was the first of many buckets does little to illustrate the sheer volume of concrete needed to construct Dworshak Dam.
The materials for the concrete were quarried from one of the granite-gneiss cliffs near the construction site. Instead of trucking the tons the rock to a crushing plant, an innovative method was devised to simplify the process. Contractors tunneled into the mountain, creating a vertical chute and carving out a cave. Inside this cave, which was about 87 feet long, 34 feet wide and 102 feet high, they installed the crushing plant.
When rock was dumped into the chute, it fell into the chamber and slid into the feeder for the crushing plant. The chute was dug 420 feet down, however, as the quarry was excavated lower and lower, that distance became shorter and shorter until the opening was only about 100 feet above the crushing chamber. The crushing chamber sat at about the same elevation as the top of the dam would be. Crushed rock was transported by a series of conveyor belts to a concrete plant that was constructed on the right abutment of the dam. In this way, concrete production for Dworshak Dam was streamlined.
As the concrete was poured, it was measured in cubic yards. Landmarks in Dworshak’s construction were marked every time a million cubic yards were poured. Contractors poured the first million cubic yards of concrete by May 28, 1969. By October 24, there were two million cubic yard of concrete poured.
By the end of construction, in the summer of 1973, 6.6 million cubic yards of concrete had been poured.
While Dworshak was built mainly for flood control, a powerhouse was constructed at its base, in a separate structure. Though small, this powerhouse contains the largest hydropower generator in USACE, rated at 253,000-kilowatts.
As with the Walla Walla District dams on the Snake River, the powerhouse was built with six slots for generator units. Three units were installed by 1972. Unlike the Snake River dams, the other three slots in the Dworshak powerhouse have never been filled.
There are two 103,000-kilowatt units in addition to the 253,000-kilowatt unit, giving the powerhouse a total capacity of 459 megawatts.
Cooling the river
Historically, salmon and steelhead migrated up the North Fork of the Clearwater River to spawn. Today, Dworshak Dam is the endpoint of that migration.
To mitigate the loss of spawning habitat, USACE constructed the Dworshak National Fish Hatchery. Completed in 1969, the fish hatchery was initially designed and constructed with a sole focus on steelhead. Salmon production was added to the hatchery in the 1980s and the hatchery now raises millions of salmon and steelhead smolts every year. These juveniles are released into the river and travel to the ocean. They return one to three years later as adults.
Because of its deep reservoir, Dworshak Dam has become a tool to aid fish migration on the Snake River. In the spring, when the hatchery releases juveniles into the river, flows from Dworshak are used to help them on their way to the ocean. In the summer, reservoir water is used to cool the river for adults migrating back upstream.
When Dworshak was built, engineers designed systems to release water from deep in the reservoir, where the water is much cooler. This water is used to keep the Snake River below 68 degrees Fahrenheit. When the cold water from Dworshak reaches Lower Granite Lock and Dam, it is pumped into the fish ladders. This process is repeated at Little Goose Lock and Dam, providing a cold-water corridor for adult fish travelling upstream in the summer heat.
Managing the lands
The Dworshak reservoir is about 54 miles long. When it was filled, it flooded about 15,000 acres, which included habitat for the local Rocky Mountain elk and white-tailed deer populations. This area has now been converted into about 3.5 million acre-feet of water storage, 2 million of which are used for regional flood control. However, this does not address the needs of the local wildlife.
To offset the loss of winter range habitat for elk and deer, the Walla Walla District now manages 9,000 acres of mitigation land to provide that winter range.
In addition to wildlife habitat, there are also lands designated for recreation. Every year, visitors flock to the Dworshak reservoir for boating, camping, hunting, fishing, hiking and picnicking. The 184 miles of shoreline now include a little something for everyone, from campsites, to trails, to boat docks.
Over the last several years, destination docks have been installed at strategic points of the reservoir to create additional locations for visitors and families to gather. The Dworshak Project is also the only place in the Walla Walla District to develop a system of ATV trails.
Celebrating tomorrow
Dworshak Dam was dedicated on June 15, 1973. The ceremony became a major event, attended by at least 2,500 people. Speeches were given by Howard H. Callaway, Secretary of the Army; Cecil B. Andrus, Governor of Idaho; and Major General Andrew P. Rollins, US Army Deputy Chief of Engineers.
Dworshak was heralded as proof that the US Army Corps of Engineers could handle any project large or small and it remains an impressive engineering feat to this day.
In June of 2023, Dworshak celebrated its 50th Anniversary. The dam remains one of the significant achievements of the Walla Walla District, which celebrates a milestone anniversary of its own, its 75th, on November 1. Between its mission in flood control and its role in providing cool water for fish passage, Dworshak is an important part of the management of the Snake River Basin.
| Date Taken: | 07.18.2023 |
| Date Posted: | 12.01.2023 15:42 |
| Story ID: | 458924 |
| Location: | IDAHO, US |
| Web Views: | 193 |
| Downloads: | 0 |
This work, Larger than life: A history of Dworshak Dam, by Hannah Mitchell, identified by DVIDS, must comply with the restrictions shown on https://www.dvidshub.net/about/copyright.
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