State’s Chehalis Basin office outlines six long‑term flood and fish strategy options, including flow‑through dam and Skookumchuck Dam choices

The Office of the Chehalis Basin on Wednesday briefed Thurston County commissioners on six long‑term strategy options to reduce flood damage and restore aquatic habitat across the Chehalis Basin, including a proposed “flow‑through” dam, a package of local levees and alternatives for the Skookumchuck Dam.

The Office’s director, Jim Zenk, said the office — housed in the Washington Department of Ecology — is charged by the Legislature to develop a strategy that balances near‑term flood protection and long‑term aquatic species restoration. “Our primary goal is to aggressively pursue flood damage reduction and aquatic species restoration within the basin,” Zenk said.

Why it matters: the Chehalis Basin is the state’s second‑largest basin and supports timber, agriculture and other economic activity; the basin has seen declining salmon runs and increasingly frequent flooding that has caused major damage in the past. The board and basin partners are weighing multi‑decade investments that could affect communities in Thurston County and neighboring jurisdictions.

Most of Zenk’s presentation described a suite of six options that technical teams are evaluating. On the structural side, one option centers on a proposed flow‑through dam upstream of the basin that would remain open during normal conditions and close only to capture predicted catastrophic flood flows; the concept includes temporary fish transport when gates are closed. That proposal is undergoing state environmental review by Ecology, which Zenk said will release a draft environmental review in November; the U.S. Army Corps of Engineers is also conducting an environmental impact statement expected next spring.

A second structural approach is a ‘‘non‑dam’’ local actions package of levees and flood walls around Centralia and Chehalis, plus levees along the Skookumchuck River to protect communities that would receive limited benefit from the upstream dam. Zenk emphasized that the options remain conceptual and would require further permitting and environmental review.

The board also discussed the future of the Skookumchuck Dam, built in 1970 without fish passage and owned by TransAlta. TransAlta is scheduled to retire its last turbine this year, prompting the basin board to study three choices for the facility: do nothing, modify the dam to improve fish passage and limited flood benefit, or remove the dam entirely. Zenk said TransAlta currently holds the water rights associated with the reservoir and that board staff are examining off‑channel storage and potential purchases to preserve those rights. “The water that’s stored behind the dam is owned by TransAlta,” Zenk said. He also said one preliminary estimate for the water rights value is about $80 million.

Participants raised tradeoffs. Commissioners asked whether a single large dam would address floods driven by different rainfall origins; Zenk agreed a dam would be most effective for floods originating in the upstream Willapa Hills, but less so for Skookumchuck‑driven events and cautioned that upstream actions could shift flooding downstream. He also noted agencies and tribes differ on the Skookumchuck Dam: Washington Department of Fish and Wildlife has urged full removal to help spring Chinook, while the Chehalis Tribe has expressed concerns about sediment and flow changes.

The office said the basin has already invested in more than 140 projects in recent years that together protect more than 300 acres and more than 200 homes and businesses; the Legislature has appropriated roughly $70 million across two prior biennia and another $75 million in the most recent biennium. Zenk said an estimate to fully implement habitat restoration in all critical areas could approach $1 billion, if every identified project were built.

The basin board intends to use technical comparative analysis to estimate how each package reduces flood risk, protects homes and affects salmon, steelhead and other species. Zenk said the board’s final long‑term strategy likely will combine pieces of multiple options rather than pick a single preset package.

The office is conducting public outreach and scheduled two in‑person open houses and an online open house: Sept. 25 at the Veterans Memorial Museum in Centralia (5–7:30 p.m.) and Oct. 8 in Montesano, plus an online open house mirroring the in‑person information. Zenk said the board plans to produce a final long‑term strategy in 2026 and will seek further public comment before legislative sessions.

The Thurston County commissioners who heard the presentation described the briefing as useful background before the county considers any resolution or formal policy positions. Commissioners pressed staff to ensure outreach to downstream water rights holders, to clarify liability and management options if ownership changes, and to bring additional financial and liability analysis back to the basin board for consideration.

Looking ahead: the Draft Environmental Impact Statement for the flow‑through dam is due from Ecology in November; the U.S. Army Corps of Engineers expects an EIS in spring. The basin board will use comparative modeling and public input through 2026 to select or combine long‑term options.