By Patrick McCarty
After a decades-long effort to relicense the 125-MW Cushman complex in Washington, northwestern utility Tacoma Power was faced with reintroducing and restoring fish populations in the North Fork Skokomish River as part of an agreement with the Skokomish Tribe and permitting agencies. The challenge was significant, but Tacoma Power’s success has received national recognition and provides a unique model for other fish passage projects.
Cushman and Little Falls
Considered an engineering marvel during the time of its construction in the 1920s, the Cushman Hydroelectric Project consists of several major components: 44-MW Cushman No. 1 and 81-MW Cushman No. 2 – each of which has its own powerhouse and dam.
Dam No. 1 impounds Lake Cushman, with downstream water being impounded by Dam No. 2 as Lake Kokanee. Meanwhile, some of the discharge from Dam No. 2 is sent back into the North Fork Skokomish River, while the remainder is funneled through Powerhouse No. 2 before exiting into Hood Canal.
Located about 2 miles downstream from Dam No. 2 on the North Fork Skokomish River is Little Falls – an area recognized as a Traditional Cultural Property of the Skokomish Tribe due to its significance as both a fishing and hunting base. The flow of the North Fork Skokomish is divided into two channels at Little Falls, both of which were previously identified as barriers to fish.
Identifying the problem
Damming of the river had long been a point of contention with the Skokomish people, who argued the obstacles for fish and constricted access created by the Cushman dams affected their traditional way of life since their completion.
The debate was in large part responsible for a 36-year-long struggle to renew Cushman’s Federal Energy Regulatory Commission license, which had expired in 1998. The relicensing process finally ended with a settlement in 2010 between Tacoma Power, the Skokomish Tribe and a number of state and federal resource agencies, including the U.S. Fish & Wildlife Service, Bureau of Indian Affairs, U.S. Forest Service, National Marine Fisheries Service, Washington Department of Fish & Wildlife and Washington Department of Ecology.
Since the settlement, the partnership between Tacoma Power and the Skokomish Tribe has grown strong. They have worked together restoring the Skokomish River Estuary, monitoring and evaluating the fish and habitat in the Skokomish River basin and continue to meet regularly to work cooperatively to design and construct the new fish facilities at Cushman.
The agreement called for the construction of two fish hatcheries, an adult fish collection and transportation system at Dam No. 2 integrated with a new 3.6 MW powerhouse, and a floating juvenile fish collector in Lake Cushman directly behind Dam No. 1.
The license anticipated the possibility that migrating salmon and steelhead may not surmount Little Falls under the flow conditions agreed to in the license. However, in order to successfully achieve programmatic goals it would be necessary for fish to successfully ascend the falls and continue to the collector. Early studies immediately following implementation of the new flows confirmed that Little Falls would need to be modified for fish to complete the journey all the way to the base of Cushman Dam No. 2.
Executing the solution
To ease passage for migrating fish and to keep the natural beauty of the culturally significant Little Falls location, Tacoma Power and the Skokomish Tribe decided to carve a fish ladder into the existing bedrock. Resting pools, bedrock weirs and connecting chutes allow the fish to pass the 12-foot-high falls in a series of bursts.
However, the modifications were complicated by a lack of accessibility, geomorphology of the bedrock, and, most importantly, the unknowns of excavating a fish passage project from natural materials. Attempting to improve passage while maintaining the nationally registered Traditional Cultural Property designation for the Skokomish people also posed a number of challenges.
The goal of preserving Little Falls’ designation led to some creative construction methods in completing the project. Building a foot path into the canyon, setting up a diesel generator above the canyon, running a 1,000-foot electric cable to the site, helicoptering in an electric air compressor, storage containers, drills and hand tools was just the start of the project. Drilling holes to blast the rock and break it into manageable sizes was used during the first half of construction. However, a blast that damaged the side channel and inconsistencies in blasting and the deterioration of the rock caused a re-evaluation of the appropriate construction techniques.
|Instead of constructing a separate fish passage system at Little Falls, Tacoma Power constructed a first-of-its-kind route directly into the area’s bedrock.|
All parties then agreed to complete the remainder of the side channel and the entire main channel using rock drills and chipping guns to remove the rock more delicately. This caused the removal to be a much more tedious project than anticipated, especially given the difficulty of accessing the site. The export of spoils also required the use of a helicopter after excavated rock was loaded by hand into 1 cubic yard sacks. The change lengthened the project’s timeline, but it allowed Tacoma Power to make the alterations needed without damaging the remaining rock. Even with the setbacks, Tacoma Power adapted to the changes and completed the project in less than four months, within the time frame allowed by permits for in water work, and in only one season although permits allowed for two years of construction if needed.
Following completion of the fish passage project, Tacoma Power proposed multiple solutions to the Skokomish Tribe and resource agencies to restore the lower side of the channel that had been damaged earlier by blasting. All agreed that rebuilding the lowest pool of the side channel using 1-yard sacks filled with excavated material would temporarily repair the channel so that it could be analyzed for a permanent fix. The temporary repair illustrated that fish can successfully pass the modified falls as nearly 100 coho salmon were documented by biologists upstream of Little Falls for the first time since the construction of Cushman Dam No. 2. The temporary structure was later destroyed in a winter flood release. A design emulating the hydraulic features of the temporary structure has been completed. The unmodified channel is currently under evaluation to determine the benefits of this potential permanent installation. Tacoma Power, the Skokomish Tribe and the Cushman Fisheries and Habitat Committee will continue to work together to develop a long-term solution.
Other than Little Falls, Tacoma Power is unaware of a fish passage structure created entirely out of existing bedrock. In most modern applications, it is customary to artificially create fish passage structures, although they often diminish the aesthetics of natural waterways. By using existing rock to create a more passable route for fish, the utility was able to maintain the integrity of the falls for the Skokomish Tribe while also creating a fish passage route for salmon and steelhead.
Both channels are currently open for passage and have successfully passed migrating fish – the first of which were observed above Little Falls in October 2014. Biologists are collecting information about the fish passage effectiveness and hydraulics of the falls. Additional studies will be performed over the next several years to determine which species use which path through the falls.
With the fish passage improvements at Lake Cushman, migrating salmon and steelhead can return to the Skokomish River and the population will continue to recover. The Skokomish people will once again be able to return to Little Falls and fish the river as their ancestors did.
Tacoma Power’s collaboration with the Skokomish Tribe was recognized by the National Hydropower Association in 2015, when the utility was given an Outstanding Stewards of America’s Water award for easing fish passage and preserving the beauty of the culturally significant location.
Pat McCarty is the generation manager for Tacoma Power.
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