By Hannah Mitchell, U.S. Army Corps of Engineers, Walla Walla District

Lower Monumental Lock and Dam sits on a remote stretch of the Snake River, in a landscape of wheat fields and rolling hills. The only town within a 20-minute drive is Kahlotus, Wash., U.S., 6 miles north, with a population of less than 200. The only major road is State Highway 261, which crosses the river at Lyons Ferry, 18 miles upriver.

Before the dam was built, the region was even harder to access.

A plan for the Snake River

At the beginning of the 1900s, there was a drive to harness the rivers of the Northwest for navigation and hydropower. To this end, the River and Harbor Act of 1916 directed the examination of the lower Snake River for a series of locks and dams.

In June 1938, the work culminated with House Document 704, 75th Congress, 3rd Session. This report recommended, among other things, the construction of four dams on the lower Snake River.

Construction of these four dams was authorized by the River and Harbor Act of 1945. However, in 1949, there was a counter-proposal.

On Nov. 1, 1948, the U.S. Army Corps of Engineers established a district in Walla Walla, Wash. This new district was given the task of building four dams on the lower Snake River. Col. William Whipple, Walla Walla District’s first commander, looked at the plan and suggested perhaps they only needed to build three.

Lower Monumental Dam, he said, could be moved downstream about 20 miles and built to be 300 ft tall, three times as tall as planned. This larger scale would allow for greater power production, flood control and irrigation. However, this plan would make it difficult to move fish, namely salmon and steelhead, from one side of the dam to the other. Barges would also have difficulty bypassing the dam and would require a multiple-step navigation lock.

In the end, the district stuck with the original four-dam plan. Lower Monumental would share the middle section of the lower Snake River with Little Goose Dam.

Sharing the wealth

In 1962, the Walla Walla District was finishing up Ice Harbor Lock and Dam, the first dam on the lower Snake River. They were also constructing John Day Lock and Dam on the Lower Columbia River, which would be given to the USACE Portland District to operate and maintain.

There were three more dams to build on the Snake River, and while the Walla Walla District shouldered this huge task, the USACE Seattle District was experiencing a lack of construction work. To share the load, the Walla Walla District handed construction of the Lower Monumental project to its neighboring district in July 1962. The Walla Walla District had completed the design, awarded supply contracts and begun construction on the first cofferdam. From here, the Seattle District would have full oversight of construction. The Walla Walla District would continue to provide real estate and design support, but the dam would be built by Seattle District engineers.

Structurally, Lower Monumental would be similar to its older sister, Ice Harbor. Like Ice Harbor, Lower Monumental was given the name of a local landmark, in this case, one of the basalt “monuments” common to the region. “Monumental Rock” was formerly named “Ship Rock” by Lewis and Clark.

Monumental Rock

Floods and setbacks

However, while Ice Harbor and Lower Monumental would have many similarities, the construction process for Lower Monumental would come with its own challenges and considerations.

During construction of Lower Monumental, the Snake River flooded multiple times. The first time was in December 1964, then in January and April of 1965. The flooding severely damaged Lower Monumental’s cofferdam during each event, with a total estimated loss of $6 million. Fortunately, these events did not result in injuries.

Lower Monumental under construction in 1965

In the spring of 1967, high waters in the Snake River threatened construction again. The contractor opened the navigation lock in response, an attempt to divert part of the river’s flow and prevent damages. The strategy worked, and construction was able to continue.

Overall, flood events on the Snake River delayed the completion of Lower Monumental Lock and Dam by about a year.

Marmes Rockshelter

While USACE engineers were building Lower Monumental, archeologists were excavating upriver at a site known as Marmes Rockshelter. This site contained artifacts from 10,000 years of ancient human history, but the Lower Monumental reservoir would soon cover the site in water.

The reservoir was scheduled to be filled in December 1968. This would allow engineers to test the dam’s fish passage equipment before the spring fish runs. However, on Oct. 31 of that year, President Lyndon Johnson ordered USACE to build a levee around the Marmes Rockshelter to protect it.

The Walla Walla District designed the levee, and the Seattle District supervised the construction. They delayed filling the reservoir until February 1969 to accommodate construction.

When the reservoir was filled, the levee held but water began to seep through, filling the Marmes site anyway. There was not enough time to fix the problem before the spring fish runs, so the site was flooded.

Today, the Marmes Rockshelter is recognized as a National Historic Landmark, although it is still flooded. It is a place of great significance to multiple Tribes and the region.

Working for the fish

A key consideration during the design and construction of Lower Monumental was fish passage, especially that of salmon and steelhead.

Salmon are anadromous, meaning they live part of their lives in freshwater and part in saltwater. In the Pacific Northwest, salmon start their lives in the freshwater rivers of Washington, Oregon and Idaho. As they mature, they travel to the Pacific Ocean, where they grow bigger and stronger before traveling back up the rivers, sometimes over hundreds of miles, to spawn and start the cycle over again.

To accommodate the needs of migrating salmon, fish ladders were built at each dam, as well as systems to help juvenile salmon and steelhead travel downstream to the ocean. However, as the Walla Walla and Portland districts constructed dams on the Columbia and Snake rivers, they had to face a water quality challenge: nitrogen gas supersaturation.

Fish ladder at Lower Monumental

One of the most effective ways to pass juvenile fish downstream of a dam is to let them pass over the spillway. As water pours over spillways, it carries air with it. When the water plunges into the river, the gases are forced to dissolve into the water, creating supersaturated water. Excessive amounts of dissolved gases, especially nitrogen, in the water can be deadly for fish. Fish exposed to large amounts of nitrogen may develop gas bubbles in their tissues and gills, a condition known as gas bubble disease. This is the equivalent of a human getting the “bends” and can be fatal.

While building dams on the Snake River, USACE tried a few methods to reduce this supersaturation. Lower Monumental was chosen to test one of these methods.

For the experiment, engineers chose Spillway Bay 4, one of the eight spillways, and modified it to prevent water from plunging deep into the river. Engineers built a concrete deflector, or “flip lip,” to deflect the spilling water across the surface of the river and dissipate the energy. This would eliminate the pressure that leads to supersaturation.

Spillway Bay 4 was modified in 1972. Testing proved the experiment was successful in reducing dissolved nitrogen in the river. The Walla Walla District then modified five more of the spillways in 1973 and 1974. By 1977, flip lips had been installed at all Walla Walla District dams on the Snake and Columbia rivers except Ice Harbor. Deflectors were not added to Ice Harbor until 1996.

Today, USACE uses a wide range of calculations in its spill operations for fish passage. Factors such as how much water is being spilled, how much water is flowing through the powerhouse, temperature, wind conditions and barometric pressure can all affect water quality conditions for fish.

USACE manages the juvenile passage spill operations at its dams to not exceed a 125% saturation of total dissolved gas. TDG is calculated by dividing the total gas pressure by the total barometric pressure. TDG is calculated for Walla Walla District dams at multiple stations on the Snake and Columbia rivers every hour.

Although spill is helpful to pass juvenile fish downstream, too much spill can cause challenges for fish, not only because of the increased presence of dissolved gasses but because it can make it difficult for adults to navigate upstream and find the entrances to fish ladders. Too much spill can also be hard on the dams themselves. The force of the water can erode the stilling basin below the dam, and hourly spill adjustments for fish can cause increased wear and tear of spillway components.

Opening the river

At the time of Lower Monumental Dam’s construction, the only way to cross the Snake River between the river mouth and Central Ferry Bridge, 85 miles upstream, was Lyons Ferry.

Lyons Ferry, originally called Palouse Ferry, was a wooden barge rigged on an overhead cable across the river. It had no engine and relied on the river current to carry it back and forth. This ferry service began in 1859 and lasted until the Lower Monumental pool was raised in 1969.

Lyons Ferry

Recognizing the need to preserve river crossing, the State of Washington stepped in to construct the Lyons Ferry Bridge at the site of the old ferry.

Today, those crossing the Snake River can do so via Lyons Ferry Bridge, Central Ferry Bridge or one of the lower Snake River dams (except for Ice Harbor). Public dam crossings are allowed at Lower Monumental and Little Goose dams once an hour, on the half-hour, from 7:30 a.m. to 4:30 p.m., Monday through Thursday. Crossings are allowed at Lower Granite Dam every day from 7 a.m. through 5 p.m. Crossings are not allowed on federal holidays.

‘…when we get there’

The year 1969 was full of milestones for Lower Monumental Lock and Dam. The reservoir reached its full height on Feb. 26. The first vessel moved through the navigation lock on April 15. The first generator began producing power on May 28.

By January 1970, three turbine-generator units were installed, with a total capacity of 810 MW, and the project was considered complete by the middle of that year. However, unlike its district predecessors McNary and Ice Harbor, there would be no dedication ceremony or VIP visits to commemorate the completion of Lower Monumental. There would be a ceremony, but not until 1975, with the completion of Lower Granite Lock and Dam and the navigation channel between the Pacific Ocean and Lewiston, Idaho. The river highway would be the star of the eventual show.

And it was not until years later, in 1978, that Lower Monumental reservoir was given a name. Many ideas had been proposed, but Congress decided on "Lake Herbert G. West," in honor of the first managing secretary of the Inland Empire Waterways Association, a man who had been instrumental in the development of the lower Snake River dams.

The construction of Lower Monumental Lock and Dam allowed for the development of an isolated region. Today, Lower Monumental allows convenient river crossing, as well as shoreline recreational opportunities. Boating is one of the primary activities enjoyed by visitors to the Lower Monumental Project. USACE parks along Lake West include six boat ramps and multiple sites for picnicking and hiking. More than 6,900 acres of the project lands are open to public hunting.

All of this is the result of Lower Monumental Lock and Dam’s construction, and the continuing work of the Walla Walla District, which celebrated its 75th Anniversary on Nov. 1, 2023. The district continues to serve as a steward of water resources for the Snake River Basin and the nation.

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

Dams & Civil Structures