Sifting Through the Studies: Eel River Dams and Fish Passage

  Cape Horn Dam and diversion site on the Eel River.

The Eel River once flowed unobstructed from its headwaters in Mendocino County to the Pacific Ocean, supporting the third largest salmon and steelhead runs in California. Over a million fish historically spawned each year within the Eel River watershed (Source: Yoshiyama & Moyle, 2010). All that has changed over the past century, with the effects of massive logging operations, dams, and water diversions. Scott Dam and Cape Horn Dam have been major contributors to salmonid population decline in the Eel River; killing fish and blocking critical spawning and rearing habitat since their construction in the early 20th century.

These dams and their associated water diversions are part of PG&E’s Potter Valley Project, a hydroelectric project that diverts Eel River water to the Russian River. The Potter Valley Project dams present major problems for Eel River fisheries, driving pressure from invasive pikeminnow, changes in fish migration timing, and increased fish mortality, all resulting in the killing of federally and state-listed threatened and endangered salmon and steelhead.

Listed species in the Eel River are protected under the federal Endangered Species Act (ESA) and California Endangered Species Act (CESA) from “take,” the term for actions which harass, harm, or kill a listed species. Take is illegal under the ESA and CESA unless properly permitted. To the extent that PG&E had the permits that allow for “take” at the Potter Valley Project, those permits expired on April 14th of 2022 when PG&E’s license to operate the project expired. These dams and diversions continue to illegally kill threatened and endangered fish today, including Northern California summer steelhead, recently listed as endangered under the CESA.

Where “Take” Occurs
Cape Horn Dam and Van Arsdale Reservoir
Cape Horn Dam and Van Arsdale Reservoir contribute to the take of threatened and endangered salmonids. The Cape Horn Dam fish ladder is the highest and longest in the state and is a well-known site of fish mortality, which migrating salmonids struggle to navigate for several reasons. First, the fish ladder and fish hotel, a structure at the base of the fish ladder that aims to help fish find and climb the ladder with proper attraction flows, often fill up with gravel and debris after high water events. This obstruction eliminates all the benefits the fish ladder provides for upstream migration, making it much harder for fish to travel past the dam.

At the Cape Horn fish ladder, migrating fish also face predation from hungry otters (Source: McMillen Jacobs Associates, 2021). The fish ladder is like an all-you-can-eat buffet for the otters, funneling fish into a narrow passageway with nowhere to hide. The shallow and warm Van Arsdale Reservoir is great habitat for another predator species, the invasive Sacramento pikeminnow. Pikeminnow eat juvenile salmonids and compete with native fish for food and resources; they are a key factor preventing population recovery for Eel River fisheries (Source: Brown & Moyle, 1997 via Yoshiyama & Moyle, 2010).

Downstream migration is another issue at Cape Horn Dam. Even if adult fish make it upstream past the dam and predators to spawn, their offspring may not make it back downstream to continue their life cycle. A few young fish may make it downstream over the dam during high water events, but this sweepstakes-style downstream passage is unreliable at best, and the risk of injury for fish falling over the face of the dam is high. There is also no real provision for the passage of rare but important steelhead kelts, adult fish who migrate back to the ocean after spawning and return to spawn again.

The Cape Horn Dam fish ladder continues to present an obstacle to the migration (and therefore reproduction and survival) of Eel River fish. The barriers to passage and resulting high predation levels at Cape Horn Dam and Van Arsdale Reservoir mean that the facility fails to meet current federal fish passage standards as defined by the National Marine Fisheries Service (NMFS). NMFS states that “fish passage must be safe, timely, and effective. Risk of physical injury, stress to the fish, and passage delay must be minimal, and the system must be able to pass sufficient numbers of adults upstream and juveniles downstream to support a viable population” (NMFS, 2021). These standards cannot be met with the existing infrastructure at Cape Horn Dam, and current conditions are causing the illegal take of federally and state listed threatened and endangered species.

Scott Dam and Lake Pillsbury
Scott Dam and Lake Pillsbury, the reservoir created by the dam, were built 12 miles upstream of Cape Horn Dam in 1922. For the past century, the 140-foot tall Scott Dam has completely blocked migrating fish from important spawning and rearing habitat in the upper mainstem Eel River (Source: Yoshiyama & Moyle, 2010). Researchers Ronald Yoshiyama and Peter Moyle (2010) found that during periods of drought, the habitat above the dam is especially critical for salmon and steelhead population survival and reproduction. Researchers from NMFS further confirmed the value of the upper basin habitat, writing in a 2021 paper that “the upper mainstem could be an important and productive subbasin … during abnormally warm years, which are expected to increase in frequency with anthropogenic climate change.”(FitzGerald, 2021) The loss of this habitat has contributed to rapid and accelerating population decline for migratory Eel River fishes.

Bottom Line
PG&E’s Potter Valley Project dams are causing the illegal take of threatened and endangered Eel River salmonids, contributing to accelerating population decline and potential future extinction.


Brown, L. R., Moyle, P. B. (1997). Invading species in the Eel River, California: Successes, failures, and relationships with resident species. Environmental Biology of Fishes, 49, pp. 271-291.  species_in_the_Eel_River_California_Successes_failures_and_relationships_with_resident_species

FitzGerald, A. M., Boughton, D. A., Fuller, J., John, S. N., Martin, B. T., Harrison, L. R., & Mantua, N. J. (2021). Physical and biological constraints on the capacity for life-history expression of anadromous salmonids: An Eel River, California, case study. Canadian Journal of Fisheries and Aquatic Sciences, 79(7), 1023–1041.

McMillen Jacobs Associates. (2021). Cape Horn Dam fish passage improvements. Technical Memorandum. Prepared for Two-Basin Solution Partners.

National Marine Fisheries Service. (2021). California fish passage: Frequently asked questions. Endangered Species Conservation.

Yoshiyama, R. M., Moyle, P. B. (2010). Historical review of Eel River anadromous salmonids, with emphasis on Chinook salmon, coho Salmon, and steelhead. UC Davis Center for Watershed Science. Report Commissioned by California Trout.