Removal By Addition on the Skokomish River

When faced with the prospect of improving its Cushman Hydroelectric Complex, Tacoma Power was able to remove fish passage barriers while adding generating capacity with an innovative expansion project.

Patrick McCarty

Tacoma Power’s 134.6-MW Cushman Hydroelectric Complex, located on the North Fork of the Skokomish River in Washington, consists of two dams, three powerhouses, two reservoirs and 26 miles of transmission lines.

The Cushman Dam No. 1 and Cushman Dam No. 2 powerhouses produce enough clean, renewable power each year to serve 27,000 Northwest homes. The project supplies 11% of Tacoma Power’s electrical generation and has the ability to run primarily at peak generation times.

After a federal relicensing process that took more than three decades to complete, Tacoma Power was faced with improving fish passage conditions at the Cushman Hydroelectric Complex as part of an expansion. The solution chosen consists of an innovative approach that combines the added power generation with a new fish collection system.

Controversial beginnings

The North Fork of the Skokomish River has long been a place of importance for the Skokomish Indian Tribe, who are known as “People of the River,” as its water provides many uses — including hydropower.

Tacoma Power constructed the Cushman project in 1929 to meet the growing needs of the city of Tacoma. The project was controversial from the beginning because it diverted the water from the North Fork of the Skokomish River into Hood Canal, bypassing 17 miles of river. Later, as Tacoma Power sought a new Federal Energy Regulatory Commission license for the project, fish habitat restoration became a central issue as fish are an important component in the life of the Skokomish Tribe.

The ensuing dispute would last 32 years, making it one of the longest FERC relicensing processes in history.

Coming to common terms

Tacoma Power, the Skokomish Indian Tribe, and state and federal agencies — including the Washington State Department of Fish and Wildlife, Washington Department of Ecology, National Marine Fisheries Service, U.S. Fish and Wildlife Service, Forest Service and Bureau of Indian Affairs — signed a settlement agreement in 2009 that resolved a $5.8 billion damages claim filed by the Skokomish Tribe during the relicensing battle.

The bulk of the damages claim was centered on the fishery. Since Tacoma Power had blocked the North Fork of the Skokomish River with a pair of dams and significantly reduced the flows into the river, it impacted the natural fishery.

The fish collection pool is visible in its lowered position, adjacent to the completed new powerhouse and original valve house at Cushman No. 2.

The agreement prompted FERC to issue an amended version of the license previously issued in 1998. The original license, which carried a 40-year term, was replaced with one issued in July 2010 that carries a 50-year term, effective from July 1, 1998. The license was signed by the utility, tribe, and state and federal agencies after two years of mediation and negotiation.

The agreement also called for the provision of upstream and downstream fish passage, construction of two hatcheries, and wildlife mitigation and recreation improvements. Tacoma Power was required to release water into the North Fork of the Skokomish River from the base of Cushman Dam No. 2 throughout the year, with flows varying between 100 to 300 cubic feet per second to mimic historic seasonal river flow.

Overcoming four challenges at once

The Skokomish Indian Tribe and natural resource agencies requested that an adult fish collection system be located at the base of Cushman Dam No. 2 to give migrating fish the maximum migration experience and spawning area.

Meanwhile, Tacoma Power wished to recover the energy from water released flowing into the North Fork of the Skokomish River at the base of the dam. The option to construct the North Fork Powerhouse was included in the settlement agreement and amended license, though the final decision to proceed did not happen until after Tacoma Power received a $4.7 million American Recovery and Reinvestment Act award from the U.S. Department of Energy.

The two goals gave the utility four challenges as it designed and built the new powerhouse and fish facility:

  1. Using water discharged from the new turbine units to drive adult fish collection
  2. Designing a method to move fish to the top or bottom of the dam
  3. Constructing the powerhouse without dewatering the construction area
  4. Maximizing available space at the bottom and top of the dam given restraints that required judicious use of every square foot

The $28 million project was supported by DOE’s grant, with the remainder primarily financed with Clean Renewable Energy Bonds.

Solutions and innovations

The fish collection facility was intended to serve two primary purposes: the first being to trap and haul migrating adult fish upstream so they can spawn in the upper basin; and second, to provide a method and location to put smolts that are migrating out from the Upper North Fork and Lake Cushman into the Lower North Fork so they can continue their migration to the sea.

The company wanted to design a fish collection system that used water discharged from the turbine units’ draft tubes to attract migrating adult fish and lead them into a fish collection pool for holding. However, turbine suppliers consulted by the company were initially reluctant to discharge draft tubes under the fish collection pool because the concept had never been tested before. The design was so sensitive that a 1:5 scale physical model was built in a Northwest Hydraulic Consultants laboratory and used to validate the arrangement. The final design protects the Andritz-manufactured turbines from the hydraulic resonance that may be created by the pool.

To move adult fish to the top of the dam and juvenile fish to the bottom, Tacoma Power’s engineers designed a tram system to move a transport hopper from the fish collection pool to a point on the side of the dam where a jib crane could reach the hopper. The crane moves the hopper to a fish handling facility and can safely deliver fish either direction.

This map displays the infrastructure associated with Tacoma Power’s Cushman Hydroelectric Complex.

Dewatering the construction area at the base of the dam was nearly impossible because of 60 feet of rubble in the riverbed. Tacoma Power came up with a plan to build a sediment control dam with bags of spawning gravel and pump turbid water to a sediment pond. This allowed excavation to be performed in the water. The 1-cubic-yard bags of spawning gravel were added to the river after construction was complete and enhance the spawning habitat as it moves downstream.

The fish collection facility was built by a joint venture of Tri-State Construction, Inc. and Harbor Pacific Contractors, Inc., above the water on a platform and lowered into place immediately adjacent to the new powerhouse when complete. Draft tubes for each of the two turbines were lowered into the water, connected to the powerhouse and encased in concrete.

The construction site itself also presented many challenges and limiting features. Space constraints included fitting the powerhouse and adult fish collector between the existing valve house and a canyon wall. Every part of the powerhouse design was laid out with only inches of room to spare.

At the top of the dam, there is limited space between the reservoir and steep hillside. Tacoma Power designed a deck over the water to create an area for fish sorting and handling. The design includes an upper and lower deck supported by the dam, a retaining wall and a supporting column in the reservoir. The fish crane foundation is notched into the side of the dam.

Adult fish are separated, counted and marked in a new fish handling area at the top of the dam before being transported in tanks by truck to locations upstream of the Cushman dams or to one of two hatcheries planned for completion in 2015.

The control system integrates control of the turbines, generators, river outlet valve and fish facility into one automated operating system. Under normal automatic conditions, there is only one required operator input: plant flow setpoint. This feature sets the control system apart from other generator plant control systems. The system was designed by Tacoma Power with components from Allen-Bradley.

The Cushman plant’s new fish collection facility, before being lowered into the waters downstream from the No. 2 dam.


The new North Fork Skokomish Powerhouse and Fish Facility provides clean, renewable energy for 1,700 homes and upstream and downstream fish passage for coho, spring chinook, steelhead and sockeye salmon. The facilities provide for the needs of Tacoma Power ratepayers and the economic and environmental interests of the Skokomish Tribe Indian and natural resource agencies.

The new powerhouse provides an additional 3.6 MW of hydropower generated by two Francis-style turbines, while the fish collection and transportation facility reopens a stretch of the North Fork Skokomish to endangered salmon and steelhead populations for the first time in more than 90 years.

This win-win scenario is a fortunate outcome of what was a long and contentious relationship between Tacoma Power, the Skokomish Tribal Nation and natural resource agencies. The completion of the hydroelectric facility is a sign that stakeholders in the Cushman Hydroelectric Project have strengthened their relationship and are working together to enhance the watershed.

The project was recognized by the National Hydropower Association with an Outstanding Stewards of America’s Waters Award, which are given annually to organizations that demonstrate excellence in the development and operation of hydropower.

Pat McCarty, P.E., is generation manager for Tacoma Power.

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