project background

Flooding is a normal part of the Chehalis basin’s ecology, with minor flooding every two-to-five years, and a major flood every 10 years or so.

The past decades have seen intensive land use and man-made changes to river flows. And climate change has made heavy rains and flooding more common.



The term “X-year flood” describes the recurrence interval of floods. A 100-year flood has a 1 in 100, or 1%, chance of occurring in any given year. A 500-year flood has a 1 in 500, or 0.2%, chance of occurring in any given year. It’s certainly possible to have a 100-year flood after only a few years. And the fear is that the Basin will have 100-year floods more frequently in the future.


Water flow is measured in cubic feet per second (cfs) flowing past a specific point. One cfs is just under 7.5 gallons. Normal summer low flow at one gage on the Chehalis (at Doty) could be just 225 gallons a second. Normal winter high flow (at Porter) could be as much as 57,000 gallons a second but well-contained in the river.

Flooding along road with water rising up the speed limit sign.
Source: Chronicle


Lewis County:
$3,996,744 in public damages (reported by cities and taxing districts)

$1,613,774 in individual damages (reported by 141 residents and 10 businesses)

Thurston County:
$2,640,000 damages total (public infrastructure and private)

$1,400,000 in private industry damages (reported by 100 residents and two businesses)

Grays Harbor County:
$800,000+ in public damages (across 12 jurisdictions)

$3,127,145 in primary residence damages (structure and personal property)

$265,200 in business damages (structure, furnishings, inventory, etc.)

Source: Office of Chehalis Basin
Note: Preliminary numbers
Flooded houses, with water about to encroach on the airport.
Source: Office of Chehalis Basin

Flood Retention Dam

Community Feedback

Communities across the Basin have differing views on the project.

Project proponents, including the Chehalis River Flood Authority and Chehalis and Centralia City Councils, have highlighted the following potential positive outcomes:

  • Reduction in the amount of flooding for residents of Chehalis and Centralia
  • Reduction in flood depths in Chehalis, Centralia and the area from Pe Ell to just upstream of the confluence of the South Fork Chehalis River
  • More than 1,200 families protected from catastrophic flooding
  • Elimination of flooding for 43% of homes, businesses, schools and other structures— the majority of remaining structures could be protected via local projects
  • Elimination of flooding at key locations along I-5 and reduced freeway closure times

Project opponents, including the Confederated Tribes of the Chehalis Reservation, the Quinault Indian Nation and many environmental organizations, have highlighted the following potential negative outcomes:

  • Reductions in aquatic species such as spring- and fall-run Chinook salmon, Coho salmon and steelhead trout
  • Degraded habitat and river and stream water quality, including tree removal at the reservoir site, increased water temperatures up to 5-9°F and decreased levels of dissolved oxygen
  • Impacts on tribal and cultural resources, including sites of significant spiritual importance on tribal lands, in violation of federal treaties
  • Lost recreational opportunities, including 14 miles of kayaking and 13 miles of riverbank fishing
  • Lack of clear funding to build the proposed dam—estimated to cost at least $600 million or as much as $1 billion, depending on the source
  • Lost restoration opportunity: The upper Chehalis River where the dam would be built is relatively undeveloped and offers good habitat restoration opportunities, especially for spring Chinook

If the proposed project moves forward into a final environmental review and permitting stage, additional adverse impacts could be identified. The Flood District would be required to propose plans to avoid, minimize or mitigate negative impacts as requirements of obtaining permits.

The Office of Chehalis Basin and the Tribes remain committed to finding solutions for both reducing flood damage and improving aquatic species habitat.

Source: Office of Chehalis Basin

Source: Chehalis River Basin Flood Authority

Chehalis Basin Timeline: The Last 100 Years


Minor flooding usually results in minimal or no property damage, but there might be some public threat. Those will likely happen with great regularity.

A major flood (100-year) on the Chehalis River is when water is flowing at more than 38,800 cfs at the Grand Mound gauge. A catastrophic flood (500-year) is water flowing at more than 75,000 cfs.

Preliminary LAND Alternative

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