Army Corps reports shifting Toutle/Cowlitz riverbed; county, state and partners pursue dredge‑spoil options and monitoring

Susan Eugenis, Cowlitz County engineer, briefed the Board of Commissioners on findings the U.S. Army Corps presented from a 2024 bathymetric survey of the Toutle and lower Cowlitz rivers. She said the Corps is changing how it describes flood probability (for example, a former "118‑year" event described as a "0.85% chance of occurrence") and that different reaches carry different authorized levels of protection.

Eugenis described sedimentation patterns near Castle Rock and at a "pinch point" north of Kelso where bars have formed and channels have shifted. "You can see how it has changed," she said, noting cross‑section charts that show about five feet of sediment accumulation in certain locations since 2013. She said the Corps will put out a request for proposals for 2025 monitoring work and that the Corps' supplemental environmental impact statement (supplemental EIS for the Mount St. Helens project) includes a decision matrix that can trigger actions such as raising the sediment retention structure (SRS) or dredging.

Eugenis explained federal‑state‑local roles: the Army Corps can have agreements only with the state; the state historically delegated local sponsor duties to the county by interlocal agreement in the 1980s and 1990s. That local sponsor (the state as delegated to the county) finds dredge‑spoil sites, secures environmental permits, and handles fish‑management tasks; the state has historically reimbursed local costs.

On dredge spoils, Eugenis said county practice has been to maintain monitoring site easements and pay landowners annually; the state reimbursed those costs. She explained a state statutory quirk reported to the board: dredge spoils from the Toutle/Cowlitz river reach do not owe the same state royalties as material dredged from the Columbia at other locations, which has historically helped make local landowners more willing to accept dredge spoils.

County and partner agencies described limited local options for spoil placement between Kelso and Castle Rock and discussed the need for early planning for sites upstream. Commissioners and regional partners described a Rivers Mead Room work group and a collaborative convened to find practical actions in the lower Toutle and lower Cowlitz, including studying commercial uses for dredge material and possible community engagement to prioritize restoration or reuse projects.

An Ecology representative briefed the commissioners on related issues including updated floodplain management proposals, planned visits by Ecology leadership to view lower‑river sites, and ongoing water‑quality research on chemicals such as 6PPD‑quinone and PBDEs that have been implicated in fish mortality. County members said monitoring work in smaller tributaries (temperature, dissolved oxygen) is under way and that alternative restoration planning may raise project prioritization for state water‑quality restoration funds.

Why it matters: sediment buildup and channel migration change flood risk and bank erosion patterns, affect habitat, and may trigger Corps actions under the Corps' decision matrix. County officials said planning for dredge‑spoil sites, environmental permits, and funding is necessary so that, if Corps triggers are met, the county and state can act without delay.

Eugenis and other staff said they are arranging preliminary meetings with the Corps and the state to identify potential spoil sites, and that the county is discussing reauthorization and potential adjustments to delegation as the existing agreement expires in June (year referenced in meeting). The collaborative and county staff said their near‑term priorities include monitoring, evaluating reuse or commercial markets for dredge material, public engagement on restoration choices, and refining the Corps' model with updated bathymetry and gauge data.

Concerns raised during discussion included limits on available spoil sites in the Kelso–Castle Rock reach; the need to update hydrologic models with major event data; and the long lead times, permitting, and cost implications of large‑scale river work. Commissioners asked staff to keep the board updated on the Corps' decision matrix triggers, potential spoil‑site options, and any state actions affecting the county's local‑sponsor responsibilities.