Chehalis River Basin Flood Control Zone District

The project, which includes the flow-through dam for flood control and the airport levee improvements, will reduce the extent of flood damage to people, homes, businesses, farms, and transportation in the Basin. It will save hundreds of millions of dollars in damages during flood events, helping our communities bounce back more quickly and at a lower financial cost.¹

Models estimate that the facility will:

• Prevent the inundation of 3,514 acres during a major flood and 3,795 acres during a catastrophic flood.²
• Eliminate flooding for about 1,280 structures.³
• Decrease flood damage to structures by 50-72%.⁴
• Protect up to 1,800 people from displacement.⁵
• Reduce the likelihood and duration of I-5 and airport closures.⁶
• Save hundreds of millions of dollars by reducing damage to structures, farms, and businesses and preventing transportation interruptions

The flow-through dam will not stop all flooding. The purpose of this structure is to help reduce damages associated with major and catastrophic flood events.

The flow-through dam is designed to operate only during major flood events, typically caused by extreme precipitation or atmospheric rivers. A major flood event is defined by a river flow of 38,800 cubic feet per second (cfs) at the US Geological Survey (USGS) stream gauge at Grand Mound.  These events typically occur in the late fall through winter timeframe. The District is continuing to research this threshold for operating the dam and will refine operational plans as more information is gathered.

Flow rates have reached the 38,800 cfs threshold at Grand Mound three times in the past 20 years.⁷  Based on the hydrologic record, major floods currently have a 15% probability of occurring in any given year (an approximate 7-year recurrence interval) but are likely to occur more frequently in the future based on climate projections. The District is continuing to refine estimates for how often the flow-through dam will operate as part of ongoing operational refinements.

The flow-through dam can retain a reservoir in the upper Chehalis Basin of up to approximately 62,000 acre-feet of water or 20.2 billion gallons. For reference, Lake Union has a volume of about 6.5 billion gallons.

Retaining water during flood events will help protect Basin communities from flood damage by reducing the depth, extent, and duration of severe flooding.

Under the most common major flood events, the flow-through dam will retain water for approximately 32 days. Water will be released during that period at a rate that minimizes flood damage downstream while also maintaining the naturally occurring wintertime flows in that reach of the river. Water will always be flowing downstream, even while the flow-through dam for flood control is holding water in the reservoir.

No, when the flow-through dam is not operating (which is almost all of the time), fish will be free to swim upstream and downstream past the facility. The fish passage conduits are designed to mimic the river, with similar depth and velocity, so fish can comfortably move through them. These conduits also allow for sediment and debris to pass through, which minimizes the disruption of ecosystem processes and maintains the natural river dynamics. Only during flood events, when water is being retained, would an advanced trap-and-haul facility be used to pass various fish species and life stages upstream.

All construction projects impact their surrounding environment, and mitigation is required to address these impacts. This is even more true for the flow-through dam because it is designed to balance two goals: reduce flood damage in the community and support a healthy ecosystem in the Upper Chehalis River Basin. The District’s Revised Mitigation Plan presents actions that will avoid, minimize, and mitigate the impact of the flow-through dam for flood control on fish and wildlife and their habitats, and includes a Revised Vegetation Management Plan to ensure healthy streamside vegetation remains upstream of the project. The goal of the Revised Mitigation Plan is to offset impacts and improve habitat function over existing conditions, providing an ecological lift for fish and wildlife.  
Federal and state agencies will review the District’s mitigation plan. If the Revised Mitigation Plan is approved and implemented, it would, in combination with all other habitat restoration work under the larger Chehalis Basin Strategy, have Basin-wide benefits for fish, frogs, and many other native species that call the Chehalis River home.

Not only will the District be required to ensure that the flow-through dam does not change the function of the habitat for native species, but the District is committed to making sure that the river has healthier habitat than before the facility is built. The mitigation actions proposed by the District would address the potential impacts of the facility on aquatic resources and help to alleviate existing habitat factors that are thought to be limiting the health of native aquatic species. This means restoring some of the natural properties that have been degraded over time, 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 through barrier removal.

FRE stands for “Flood Retention Expandable” and is the technical name for the flow-through dam for flood control on the Chehalis River. This is how the facility is referred to in the State and National Environmental Policy Act (SEPA/NEPA) documents.

The design of the flow-through dam is considered “expandable”, meaning it includes elements developed or designed to not preclude future expansion of the facility. To be clear, the current project does not include future expansion.

No, there are no plans to expand the facility at this time. Any expansion would be a separate project, subject to federal and state approvals, tribal consultation, public review, and environmental permitting; the facility is simply being designed to not preclude expansion.

Floods in the Chehalis Basin are defined by how fast water is flowing at the U.S. Geological Survey (USGS) Grand Mound gage, measured in cubic feet per second (cfs). When this flow rate reaches 38,800 cfs, it is called a major flood, which is similar to the 2009 flood event. Major floods are sometimes called 7-year floods because that is about how often they are expected to occur. When the flow rate reaches 75,100 cfs, it is defined as a catastrophic (or 100-year) flood, which is similar to the 1996 flood. The 2007 flood was even larger than a catastrophic flood and is categorized as a 500-year flood.⁸