4. Long-Term Options
The long-term Chehalis Basin Strategy options
The Chehalis Basin Board is considering larger, longer term investments to protect the basin’s people and aquatic life for decades to come.
In June 2025, the Board met for an all-day workshop to determine what combinations of projects and investments they would like to see studied further through early 2026. Board members reached a consensus on six different long-term strategy options to evaluate – a major milestone for their process. These six long-term options:
- Represent a broad range of potential pathways for the future of the basin — from high investments in proposed structural flood damage reduction and aquatic restoration measures, to a focus on non-structural flood protection, and several “middle of the road” options in between
- Combine individual projects and investments in a way that considers how they interact with each out
- Reflect Board members’ consideration for how to gain the most information on the relative economic, social, and ecological costs and benefits
- May change based on what is learned from the technical evaluation and community input
To see the Board in action, you can watch the June 6 workshop recording (1.5-hr).
What do the long-term options propose?
Dam-and-partial-levee-system options:
- Option A: This option includes the proposed flow-through dam near Pe Ell and a portion of the levee system in and around Chehalis and Centralia (Group 1 – the North and South Skookumchuck levees only) to provide flood protection. It would also modify the structure and operations of the Skookumchuck Dam to increase downstream flood protection and provide some improvements to current fish passage, and provide a high level of investment in basin-wide aquatic restoration.
- Option B: This option is closely related to Option A, but instead of modifying the Skookumchuck Dam, it proposes removing it in order to substantially improve fish passage on the Skookumchuck River. It also includes the China to Saltzer Creek levee in addition to the North and South Skookumchuck Levees, as well as improvements to China Creek.
Levee-centered options:
- Option D: This option relies on the full system of levees, floodwalls, and drainage improvements, as well as an increased level of investment in voluntary floodproofing, elevations, and buyouts of homes and businesses. It would also remove the Skookumchuck Dam to substantially improve fish passage on the Skookumchuck River, while providing a lower investment level of basin-wide aquatic restoration.
- Option E: This option is closely related to Option D, with two key differences: it includes no changes to the Skookumchuck Dam, and high investment in basin-wide aquatic restoration.
- Option F: This option relies on a portion of the levee system: Group 1, plus the Airport Levee. It couples that partial system with a high investment in local, small-scale flood projects as well as a medium-high level of investment in voluntary floodproofing, elevations, and buyouts of floodplain structures not protected by Group 1 levees. It also includes no action on the Skookumchuck Dam and a lower investment in basin-wide restoration.
Non-structural flood protection option:
- Option C: This option is the least reliant on structural flood protection and includes no flow-through dam or levees, instead focusing on the highest level of investment in local flood projects and voluntary floodproofing, elevations, and buyouts of homes and businesses. Like Option F, it suggests no action be taken on the Skookumchuck Dam and provides a lower investment in basin-wide aquatic restoration.
A baseline “no strategy” option will also be evaluated to provide information on how the basin would be affected if OCB’s investments went away completely.
The proposed Chehalis River flow-through dam for flood control
The Chehalis River Basin Flood Control Zone District (Flood District) is seeking to construct a flow-through dam and temporary reservoir on the Chehalis River near Pe Ell and make improvements to the levee around the Chehalis-Centralia Airport. The Board is evaluating whether to include the flow-through dam as part of its recommended long-term Chehalis Basin Strategy.
Under the Flood District’s proposal, the Chehalis River would flow freely through the proposed flow-through dam structure unless a major flood is forecasted to occur in the upper Chehalis Basin, triggered by rainfall in the Willapa Hills. The dam would then close its gates to temporarily hold back floodwaters, and water would be safely released later. With this design, fish would be able to swim upstream and downstream of the facility most of the time. During a flood event when water is being retained, a trap-and-haul facility would be used to pass various fish species upstream.
Potential benefits
If constructed, the project would reduce the extent of flood damage to people, homes, businesses, farms, and transportation in the basin. Early estimates from the Flood District anticipate the flow-through dam would:
- Reduce floodwater on nearly 4,000 acres during major and catastrophic floods
- Eliminate flooding for approximately 1,300 homes and businesses
- Reduce the likelihood and duration of I-5 and airport closures
- Save hundreds of millions of dollars by reducing damage to structures, farms, and businesses and preventing transportation interruptions
Just like with the proposed levee system (expanded upon below), it will not be possible to protect all homes and businesses with the dam. However, the recommended long-term Chehalis Basin Strategy will include support from the OCB to floodproof, elevate, or relocate any structures that would still experience some flooding.
Environmental review and mitigation measures
In 2020, under the State Environmental Policy Act (SEPA) and National Environmental Policy Act (NEPA), the Washington State Department of Ecology and the U.S. Army Corps of Engineers each conducted an environmental review of the proposed project and released two separate Draft Environmental Impact Statement (DEISs). Both analyses described significant potential impacts from the construction and operation of the flow-through dam to salmon, wildlife, and the surrounding habitat, to Tribal and cultural resources, and to recreational opportunities. In 2023, to address these findings, the Flood District:
- Revised the project design to update the location, construction, operation, and fish passage features of the proposal – as shown in the images above.
- Developed a draft mitigation plan that includes a suite of actions intended to offset impacts to salmon and other aquatic species and improve habitat function – such as planting native trees and shrubs along the stream bank to increase shade and cool the water, increasing habitat complexity by creating side pools, and opening fish access to blocked habitats by removing barriers.
Ecology and U.S. Army Corps of Engineers are now conducting an environmental review of the revised project proposal. Ecology’s revised Draft EIS is expected to be released in late 2025 and will have a public comment process before the Final EIS is issued in 2026. The SEPA comment period (just related to the flow-through dam) is separate from the feedback OCB is collecting on the long-term Chehalis Basin Strategy options. If you would like to comment, visit the project’s SEPA page for more information and sign up for updates.
Why SEPA and NEPA?
The State and National Environmental Policy Acts require state and federal agencies to systematically evaluate proposed governmental decisions or projects by identifying and analyzing associated environmental impacts. This is referred to as the environmental review process. Agencies must conduct and publish a formal study, or Environmental Impact Statement (EIS), if a decision or project is determined to likely have significant adverse environmental impacts. No permit decisions can be made until after the environmental review process is complete.
Preliminary cost estimate
Based on the updated design, the Flood District estimates that the project could cost between $1.3 – $2.3 billion. This preliminary cost estimate factors in construction of the flow-through dam, levee improvements around the Chehalis-Centralia Airport, and mitigation measures. For context, damage and disruption from the 2007 flood is estimated to have cost $900 million.
The proposed system of levees and floodwalls
The proposed system of levees and floodwalls (also known as the Local Actions Non-Dam Alternative, or LAND) is a proposed system of new and expanded levees, floodwalls, and drainage improvements primarily in and around Centralia and Chehalis that is intended to reduce flood-related damage in the upper Chehalis Basin. This alternative is still in its early (approximately 10%) design phase.
As part of the long-term strategy development process, the Chehalis Basin Board is considering all, some, and no segments of the proposed project alongside other flood protection measures for the greatest level of protection, including the proposed flow-through dam.
Potential benefits
If fully constructed, the proposed flood protection structures could:
- Protect over 2,000 homes and businesses
- Maintain and enhance emergency routes and road bypasses during floods
- Create of a new recreational trail system that improves access to the river and could connect to existing and proposed park systems
- Lower flood insurance premiums for landowners
LAND is designed to have no or negligible impacts to downstream communities and to preserve aquatic habitat by providing room for the river to follow its natural process. Just like with the proposed flow-through dam, it will not be possible to protect all homes and businesses with these interventions. However, the recommended long-term Chehalis Basin Strategy will include support from the OCB to floodproof, elevate, or relocate any structures that would still experience some flooding.
Preliminary concept design
While general areas for the proposed LAND flood protection structures have been identified (shown in map), it is still too early in design to know where the exact alignments would be. The final alignments could be different based on future analysis. If the Chehalis Basin Board recommends the proposed system (all segments or a subset of them) be built, input from local cities, partners, and community members, as well as further technical studies, will help refine alignment location.
One of the goals of LAND is to develop a flood protection alternative that could be implemented fully or partially, at once or incrementally over time. To provide options for how the project could be divided, the design team developed groups of the proposed levees.
If the Board recommends any of the proposed LAND features, several more steps would need to be completed before anything could be built, such as:
- Additional technical studies to advance design and narrow down cost estimates
- Community engagement to refine location of flood protection structures and determine feasibility
- Identification of project sponsor (e.g., local government or special district) and funding sources
- Environmental review and permitting
Project Grouping
- Group 1: (1) North Skookumchuck, (2) South Skookumchuck, (3) Improved Chehalis River Conveyance, (4) Mellen Street Bridge Replacement, (5) China Creek to Salzer Creek, and China Creek Improvements
- Group 2: (6) West Skookumchuck, (7) Fort Borst
- Group 3: (8) Airport, (9) Newaukum, (10) Adna
In addition to levees and floodwalls, Group 1 includes excavation of approximately 800,000 cubic yards of material to improve flow in the Chehalis River (3), and removal and replacement of the Mellen Street Bridge (4), which currently restricts flow upstream during flood events.
Preliminary cost estimate
Based on the current design, technical experts estimate the project could cost $900 million – $1.6 billion. This preliminary cost estimate factors in permitting and construction of the levees, reconstruction of the Mellen Street Bridge, and floodproofing, raising, or relocating of homes or businesses in the area that would still experience some flooding. For context, damage and disruption from the 2007 flood is estimated to have cost $900 million.
Potential pathways for the Skookumchuck Dam
The Chehalis Basin Board is considering future pathways for the Skookumchuck Dam, including modifying or removing the dam, or taking no action at all. The Board took interest in studying the dam after learning that TransAlta will be closing its Centralia steam generation plant in 2025. The dam—which sits at the midpoint of the Skookumchuck River—has stored water for the power facility since 1970. The dam could be modified to reduce flood damage and improve fish passage. There are no current planned changes to the dam, however TransAlta is also exploring future actions that would require water. If the Board recommends modifying or removing the dam, any further action would require an agreement with TransAlta as well as additional design, environmental review, and permits.
Potential to reduce flood damage
While the Skookumchuck Dam and its four-mile-long reservoir were not designed and are not managed to reduce flooding on the Skookumchuck River, they have provided some flood benefit. When the catastrophic December 2007 flood struck the Chehalis Basin, the reservoir behind the dam was unusually low, allowing it to hold back water that would have made flooding in Bucoda and Centralia even worse. A recent technical analysis pursued by the Office of Chehalis Basin (OCB) found that modifying the dam’s structure and operations for additional flood storage could provide significant reductions to the area and depth of flooding downstream.
Opportunity to improve fish passage
While the dam allows downstream fish passage at certain reservoir levels, it prevents fish from traveling upstream. The Washington Department of Fish and Wildlife (WDFW) has occasionally captured and hauled adult steelhead upstream of the dam during spawning season. At the same time, the dam has never been operated to make it easier for juvenile fish to pass downstream. OCB investigated potential effects of changing discharges from the dam or improving the fish transport facilities. The analysis found modifying dam operations for fish passage could potentially benefit steelhead and coho salmon while removing the dam could substantially benefit steelhead and coho, as well as spring and fall Chinook, a declining species for whom the Skookumchuck River is a remaining stronghold.
Potential impacts to water rights
The dam stores water during the winter that is discharged into the river downstream during the summer, adding approximately 50 cubic feet per second (cfs) of water to the river available year-round. TransAlta currently holds the right to use that water, and has used it to power the steam generation facility. The company established a water bank in 2021 that allows it to maintain its water right and sell the right to others, once the water is no longer needed at the steam plant. Without an alternative way to store water, modifying dam operations (and especially dam removal) could lead to a reduction of water in TransAlta’s water bank. If that occurred, water right users downstream such as municipal water suppliers or some farms may also see their water availability impacted. TransAlta and other water users may have to be compensated for the lost water right. Potential options to maintain the existing water bank through off-channel water storage have also been studied.
Existing programs to be scaled up or maintained
In addition to the considerations above, the Board also seeks to continue building upon the momentum achieved across over the last decade. The long-term strategy will include continued support for local projects, policies, and programs that protect the basin’s communities and habitats. These include initiatives you just read about — Aquatic Species Restoration Program (ASRP), Flood Authority local projects, the Flood Warning System, and floodproofing, elevations, and acquisitions through the Community Flood Assistance and Resilience (CFAR) Program and other potential programs like it. Whether these investments are maintained or scaled up depends on the Board’s recommendations about the proposed flow-through dam, the proposed levee, and the Skookumchuck Dam.
The long-term Chehalis Basin Strategy will also provide resources for continued assistance to Tribes and local governments with floodplain management, land use planning, and transportation planning, as well as for two new integrated programs administered by OCB that work with willing landowners to address both goals of the strategy: the Erosion Management Program and the Multi-Benefit Acquisition Program.
