Army Corps survey shows shifting sediment in Cowlitz River; county and state discuss dredge spoils and monitoring plans

COWLITZ COUNTY — The Army Corps of Engineers’ latest bathymetric survey shows sediment bars forming in the Cowlitz River and changes in channel alignment that county staff and state partners say are reshaping local flood risk and bank erosion patterns.

Susan Eugenas, Cowlitz County engineer, summarized maps and cross sections presented at the Army Corps’ annual meeting and said the Corps is shifting away from 100‑year nomenclature toward probability metrics (for example, a “0.85% chance of occurrence,” formerly described as a 118‑year event). She said different reaches of the river have different levels of protection and that some study lines show declines in authorized levels of protection since 2022, though the lines remain above the authorized threshold.

Eugenas said the Corps’ slides show a bar that has grown north of Castle Rock, producing about 50 feet of bank erosion in that reach since 2013 and a “pinch point” near Rocky Point between Lexington and North Kelso where the channel transitions to sandier bars. “You can see how it has changed,” she said, noting a cross section that has gained roughly five feet of sediment since 2013 and that “the river is shifting to the right bank that everybody is noticing the erosion.”

County and state staff discussed the Corps’ decision matrix in the Mount St. Helens supplemental environmental impact statement (EIS). Eugenas said the Corps has a matrix that would trigger actions — such as raising the sediment retention structure (SRS) design and, at some thresholds, dredging — but she did not provide the decision matrix at the meeting, saying she would bring it to commissioners for review.

Eugenas also said the Corps will issue a request for proposals for 2025 monitoring and that the Corps has asked county staff to help identify potential spoil‑site locations if a dredging trigger is reached. She explained that, historically, the state was the Corps’ local sponsor and paid for land purchases and fish handling; the state later delegated that role to the county under an interlocal agreement. That agreement, she said, expires in June and the county has asked the state to reauthorize or resume responsibility.

The county discussed practical limits on available spoil sites between Kelso and Castle Rock and noted that, under a state law discussed at the meeting, dredged spoils from the Toutle and Cowlitz rivers south of the Toutle mouth do not owe the state royalties — a provision county staff said helped encourage property owners to accept spoils in the past.

County staff and partners also described the Rivers Need Room work group, a smaller collaborative that met recently in Longview to discuss actionable steps for the lower Toutle and Cowlitz reaches. The group includes representatives from ports, planning departments, the tribe, the Lower Columbia Estuary Partnership and the Fish Recovery Board. Participants are exploring commercial uses for dredged material and community engagement to prioritize projects that open floodplain capacity and improve habitat.

Shauna (staff member) said the collaborative is working to identify potential community projects and public engagement steps and that local leaders at the Council of Governments are exploring ways to find commercial markets for the dredge material.

Separately, state Ecology staff briefed commissioners on water‑quality and contaminant issues. Staff described recent studies into 6PPD‑quinone (a tire‑related chemical linked to coho die‑offs) and PBDEs (a class of chemicals that can bioaccumulate), noting that some findings point to treated wastewater and urban runoff as contributors. Ecology staff also said it is conducting temperature and dissolved‑oxygen monitoring on smaller tributaries and exploring alternative restoration plans to prioritize water‑quality funding.

Ending: Susan Eugenas said she will bring the Corps’ decision matrix and more detailed monitoring information to the board and that county and state staff will continue collaborative work to identify potential spoil sites, monitoring contracts and community priorities for dredge material reuse.