March 9 public forum focuses on Skookumchuck Dam study

Assessment evaluating if dam can help reduce flood damage, improve fish habitat

Our Office of Chehalis Basin (OCB) is studying whether the Skookumchuck Dam in Thurston County could help improve fish passage and habitat or reduce flood-related damage on the Skookumchuck River.

Owned and operated by TransAlta, the dam and its associated 4-mile long reservoir are located about 15 miles upstream from the city of Centralia. The feasibility assessment supports the Chehalis Basin Strategy to reduce damages from major floods, and restore and enhance aquatic species habitat in the Chehalis River Basin in Southwest Washington.

Public forum March 9

On Wednesday March 9, OCB will hold a virtual public forum from 6 to 7:30 p.m. to share information and take questions about the Skookumchuck Dam. The event will focus on the history of the dam as well as its effects on flooding, fish, and water rights in the Skookumchuck River. The forum also will include information about the ongoing dam evaluation including the feasibility to:

  • Enhance the dam structure to reduce flood risks or improve fish passage.
  • Modify current operations to enhance fish passage and habitat.
  • Remove the dam.

The public can register online to attend the webinar.

Dam study includes many partners

OCB is working with TransAlta, representatives from the cities of Bucoda and Centralia, and the Confederated Tribes of the Chehalis Reservation, as well as environmental restoration and flood experts to collect data and build models to evaluate options for the dam

“There are no immediate planned changes to the Skookumchuck Dam,” said OCB project manager Nat Kale. “Instead, we are assessing the feasibility of whether adjusting dam discharges, modifying the structure of the dam, or completely removing it will help fish or diminish flood risks.”

Dam study results ready in 2023

Completed in 1970, the dam provides water for the Centralia Steam Generation Plant. TransAlta uses water from the reservoir to operate its remaining plant turbine. OCB and the Chehalis Basin Board started studying the dam last year after learning TransAlta will close its final coal burner in 2025.

We anticipate we will be ready to share our assessment with the Chehalis Basin Board in early 2023. Upon reviewing the results, the board may request additional studies, select an option to pursue further, or take no steps. If the board decides it is interested in moving forward with a preferred option, any project will require additional design, environmental review, and permits ahead of construction.

Potential flood damage reduction benefit

While the dam is not designed or operated in a manner to help reduce flooding on the Skookumchuck River, the structure has provided some benefit. For example, when the catastrophic December 2007 flood struck the Chehalis River Basin, the reservoir behind the dam was unusually low, allowing it to hold back water that would have otherwise made flooding in Bucoda and Centralia even worse. The current assessment will build on a 2012 U.S. Army Corps of Engineers study to examine how this partial, unintended flood reduction benefit might be improved.

Fish passage limited

Because the dam prevents fish from traveling upstream, the former Washington Department of Fisheries required the owners to build a fish hatchery and an associated fish capture and transportation facility to offset fish habitat lost to the dam.

Until 2008, the only fish to travel upstream past the dam were steelhead the state Department of Fish and Wildlife captured and transported during spawning season. Between 2008 and 2020, no fish were transported upstream of the dam. In 2020, WDFW started trapping and hauling steelhead above the dam again.

Our study will investigate whether changing discharges from the dam, improving the 50-year-old fish capture and transport facilities, or fully removing the dam will benefit fish in the river.

Potential water right impacts

The dam stores water during the winter that is then discharged during summer months. This creates approximately 60 cubic feet per second of water that is available year-round. TransAlta currently holds the right to use the water.

TransAlta used its water right to power two electrical power generation turbines until 2020, when it shut one of them down. TransAlta continues to use water to operate its remaining turbine. Although TransAlta plans to shut down the remaining plant turbine in 2025, the company established a water bank in 2021, allowing the company to maintain its right to beneficially use the water and to sell the right to others.

Should the dam be removed or altered in a way that keeps TransAlta from storing enough water, there would be a reduction of water in the water bank for TransAlta to sell. Downstream water users, such as municipal water suppliers or farms relying on surface water irrigation, may also see their water availability affected. If that happens, TransAlta and other water users may have to be compensated for impacts to water rights.