Sifting Through the Studies: Habitat Behind Scott Dam

Upper Eel River Habitat: cold water, deep pools, and places to hide from predators.

Chinook salmon and steelhead can’t reach hundreds of miles of their former habitat because the Eel River is blocked by Scott Dam. According to the latest research, this upper basin habitat provides ample cold water habitat, even in warm years. So Eel River dam removal is a huge opportunity for salmon and steelhead recovery.

A key paper recently released by National Marine Fisheries Service scientists looked at how well the reach of the upper Eel River blocked by Scott Dam would support native runs of salmon and steelhead today. Scientists compared water temperatures, physical characteristics of the river, and presence of invasive species to what fish need during every life stage. They found the upper basin offers salmon and steelhead a real chance for recovery.

The high-quality habitat in the Upper Eel River could give multiple runs of salmon and steelhead an opportunity they haven’t had in 100 years. The federal biologists concluded, “the blocked Upper Basin has a higher proportion of suitable habitat for all freshwater salmonid life stages than much of the rest of the Eel River Basin.”

Characteristics of “suitable habitat” in the Upper Eel

Root wads and fallen trees, boulders and cobbles, deep pools, and riffles all provide places for juvenile salmon and steelhead to hide and find food. Shaded creeksides, forest streams, and widened riffles harbor the tiny insects which are juvenile salmonids’ main source of food. Researchers found nearly all of the upper Eel above Scott Dam offers fair or better spawning and incubation habitat in physical terms. But that’s true of the rest of the Eel River basin below the dams. What makes the upper basin’s streams such high-quality habitat for salmon and steelhead is the cold water they hold, even in the warmest years.

Optimal temperatures for steelhead and Chinook life stages range from 6 – 20 degrees C (42.8 – 68 degrees F). Salmonids are especially sensitive to high temperatures during their early life stages – incubation, emergence, and rearing. Federal researchers found stream temperatures in the Upper Mainstem were cooler than the rest of the Eel watershed year-round, even during warm years.

Cold water means safe refuge

The invasive Sacramento pikeminnow was introduced to the Eel River via Scott dam’s Lake Pillsbury reservoir, where it was used as a baitfish. Pikeminnow are voracious predators on young salmon and especially steelhead. However, pikeminnow cannot tolerate the cold water temperatures optimal for steelhead and Chinook. The coldest temperature that pikeminnow can tolerate is 18 degrees C (66.4 degrees F). Researchers found the cold waters of the upper mainstem would likely provide significant refuge from pikeminnow predation for juvenile salmon and steelhead.

Removing Cape Horn and Scott dams would thus give adult steelhead and Chinook salmon passage into the clean, cold water habitat where their young thrive, but their predators don’t.

What about climate change?

As the climate crisis worsens and temperatures continue to rise, the upper Eel may well become even more important to our salmon and steelhead. The National Marine Fisheries Service determined that

“… the Upper Mainstem could be an important and productive subbasin for the Eel River Basin during abnormally warm years, which are expected to increase in frequency with anthropogenic climate change.”


Bottom Line
The habitat blocked by Scott dam has cold water even during warm years and offers one of the best opportunities for recovery of salmon and steelhead in the entire Eel River Basin.


Dr. Alyssa M FitzGerald, Dr. David Boughton, Mr. Joshua Fuller, Ms. Sara N John, Dr. Benjamin T. Martin, Dr. Lee R. Harrison, and Dr. Nathan J Mantua. 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.