‘Big One’ concerns could prompt lower water levels at Detroit Lake

Oregon

Dam in "very good condition," but Army Corps wants to mitigate risk in case of worst-case scenario

CLACKAMAS COUNTY, Ore. (KOIN) — The U.S. Army Corps of Engineers will lower summertime water levels in Detroit Reservoir in order to mitigate risk in the unlikely event of a large earthquake, a decision residents and businesses worry will shorten the recreational season.

Detroit Dam is located about 150 miles east of the Cascadia Subduction Zone, a megathrust fault that stretches from Cape Mendocino, California all the way up to Vancouver Island, Canada. The subduction zone is capable of producing magnitude 9 and larger earthquakes approximately every 300 to 500 years.

While Corps officials stress that the dam is in “very good condition,” a seismic hazard analysis last year revealed there is a possibility the spillway gate’s supporting arms could buckle during such an event, resulting in an uncontrolled release of water. If that happened, it could unleash devastating flooding on large swaths of the North Santiam River canyon and urban areas, including the state capital.

“You’re looking at the entire watershed downstream,” said Dustin Bengtson, Willamette Valley Project deputy operations project manager for the Corps.

The potential threat to life and property is why Bengston said the Corps needs to err on the side of caution and take immediate action: Lowering the maximum lake level by 5 feet.

Detroit Lake normally fills to 1,563.5 feet at the dam. The Corps plans to lower that level to 1,558.5 feet, which would put less pressure on the dam in the event of a major earthquake. The reduction will likely remain in place for several years as the Corps studies the plan further and determines long term steps, which could include reconstruction of the spillway structure if needed.

Five feet may not seem like a lot, but it’s enough to cut the lake’s recreation season short. And the lake is the center of Detroit’s economy.

detroit lake marina 07192015_182196
Detroit Lake Marina as seen in July 2015 (KOIN file)

“Our businesses really only have five months to make it, from about April through the middle of September, when the lake level comes up and then when it goes down,” Detroit Mayor Jim Trett told KOIN 6 News. “So any kind of shortening of that season tremendously impacts businesses because it just limits what recreational opportunities we have.”

The Corps does not expect the reduction to impact water supply, only recreation.

Filling Detroit Lake has already proven difficult for the past decade, Trett said, leading to several summers where the season was cut short.

Trett was warned about the Corps’ plans last week. When the news became public Monday, residents proceeded to call the mayor en masse concerned primarily about the lake level and its impact on the economy, he said.

The community is still reeling from the impacts of the coronavirus pandemic, followed by the September 2020 wildfires, which scorched much of the town.

Trett said one of the marina owners told him they plan to open and do the best they can, but “it’s just another poke in the eye for us and for the community.”

The Corps acknowledged the Santiam Canyon is going through a difficult time and that the decision to lower the water level will have impacts.

“This decision’s being made with life safety as a paramount factor and that is our first priority is protecting the life safety risks associated with our dams,” Dam Safety Program Manager Ross Hiner said.

Public invited to weigh in on plans

The public can weigh in on the Corps’ plans. Comments will be accepted until March 30 and can be emailed to DetroitPoolRestrictions@usace.army.mil.

The agency is also holding a virtual meeting to talk about the proposal at 5 p.m. March 22 on WebEx. Click here to see the presentation when it happens. Anyone who is unable to join to web meeting can call in using to toll free number (844) 800-2712, access code 199 430 8599. Public comment will not be taken during the meeting.

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