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‑loc…