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Army Corps survey shows shifting sediment in Cowlitz River; county and state discuss dredge spoils and monitoring plans
Summary
County engineer reported Army Corps bathymetric results showing sediment accumulation and channel migration in the Cowlitz River. County staff and state partners discussed monitoring, potential dredging triggers identified in a supplemental EIS, the need for dredge‑spoil sites, and collaborative work to find commercial uses for dredged material.
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…
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