Heat Isn’t the Only Thing That Could Kill ‘Nearly All’ Young Salmon in the Sacramento River

A person holding a salmon.

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Photo: Steve Martarano/U.S. Fish and Wildlife Service (AP)

On Tuesday, CNN reported that “extreme heat” from California’s searing temperatures could kill “nearly all juvenile chinook salmon” in the Sacramento River. But the potential grim fate of the salmon isn’t just due to climate change—human meddling in California’s rivers is also to blame.

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This year’s extreme heat, which has seen all-time records fall across the West, is playing a role in the salmon crisis. But to truly understand the sad salmon story, you need to go back to the early 1940s when the Shasta Reservoir, the largest reservoir in California, was formed by damming the Sacramento River. The manmade lake is the centerpiece of the system of dams, canals, and pumps called the Central Valley Project, a vast network that supplies water to 29 of the state’s 58 counties. That includes large amounts of water used by the sprawling agriculture industry in the Central Valley.

When the reservoir was built, however, it blocked access for many salmon runs to colder mountain streams where the fish traditionally spawned. Salmon eggs need pretty chilly river water to incubate and thrive during their spawning period: anything above the threshold of 56 degrees Fahrenheit (13.3 degrees Celsius) is bad news for the eggs, which start to die off in warmer water. Since the reservoir’s construction, ensuring that there’s enough cold water in the Sacramento River during spawning season—especially for the endangered winter-run Chinook salmon that spawn from June to September—has been part of the goal of managing the reservoir, a task that falls to the federal Bureau of Reclamation.

Keeping the water in the Sacramento River cold, in turn, means making sure that the Shasta Reservoir stays pretty full. “If you think of a bathtub, the top layer warms up because of the sun and the exposure to the air, and the bottom layer is colder,” said Kate Poole, a senior attorney with the Natural Resources Defense Council. “You want to run Shasta to maintain a certain level of water so that that coldwater pool at the bottom is big enough to provide cold water for that whole spawning and rearing season.”

But that’s much easier said than done in a punishing drought and a searing series of heatwaves, as the state grapples with a dwindling water supply. “The reservoir has been drained so much [this year] that they have run out of cold water,” said Poole.

“It’s an extreme set of cascading climate events pushing us into this crisis situation,” Jordan Traverso, a spokesperson for the California Department of Fish and Wildlife, said in an email.

Traverso said that the record heat in California this month is making life harder for the salmon as it puts pressure on the already-dwindling cold water supply in the Shasta Reservoir. “This persistent heat dome over the West Coast will likely result in earlier loss of ability to provide cool water and subsequently, it is possible that all in-river juveniles will not survive this season,” the agency told CNN in a statement.

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“There’s still a little bit of cold water left now, but the alarm CDFW is raising is that it’s depleting a lot faster than [the Bureau of Reclamation] said they would,” Poole said. “They’re pumping water out too fast so there’s a danger for all the juvenile salmon.”

The salmon are unfortunate victims of a statewide crisis over water use in the midst of a megadrought fueled by the climate crisis. A lot of the demand for the water from the Shasta Reservoir that the Bureau of Reclamation is responding to is from farmers, who have historic claims to a certain amount of water from the Sacramento River. (Poole said rice farmers in the Central Valley are a major recipient of water from the reservoir.) Even as the salmon struggle, many farmers across the state are also tightening their water belts as they receive limited supplies of water, with some making tough choices to tear up water-thirsty crops like almond trees. California Gov. Gavin Newsom recently called on citizens across the state, from farmers to homeowners, to conserve their water use by 15%.

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These aren’t the only salmon fighting for their lives. Elsewhere, CDFW has come up with an audacious plan to truck 17 million young salmon to the sea. The agency believed the fish wouldn’t otherwise be able to traverse rivers running too hot because low water levels have allowed water to reach temperatures levels dangerous to the fish’s survival.

This also isn’t the first time that a statewide drought, extreme water demand, and excessive heat have combined to spell trouble for the salmon. During the state’s major drought in the mid-2010s, Poole said, around 80% of the winter-run Chinook eggs were wiped out due to high temperatures in the Sacramento River. It was a dangerously close call for the salmon, which have a three-year lifespan.

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“What we and climate scientists have been saying [to the Bureau of Reclamation] since then is, ‘hey, you guys need to expect hotter air temperatures, you need to expect less snowpack and more frequent drought,’” said Poole. “Let’s plan for this and adjust the system so that we take those things into account. That’s what Reclamation has not done. They haven’t changed their operations to deal with those changing realities. It’s all this very skewed system that is falling on the backs of salmon.”

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