By Robin Meadows
While marine heat waves are well known, a 2022 study was the first to document aquatic temperature spikes in rivers nationwide. Even river experts were surprised.
“I’d never thought about it,” recalls Sarah Null, a Utah State University physical geographer who focuses on environmental water management. “But when I saw the paper, I thought ‘of course.’”
River heat waves are temperature extremes that last several days or more. “They’re out of the ordinary—often 5 to 10 °C warmer than normal for the time of year and place,” says Jonathan Walter, a UC Davis quantitative ecologist who co-authored the 2022 nationwide study. Warm water can stress or even kill aquatic life. Fish that depend on cold water, such as salmon, steelhead and cutthroat trout, are particularly at risk.
Now Walter, Null and colleagues are taking a deeper dive into river heat waves in California and eight other Western states. The nationwide study had “huge gaps,” Null says. “We’ll fill in those gaps in the West.”
The researchers are analyzing 15-plus years of continuous temperature data at 118 river sites across the region. While the previous study had just one California site, the new one has 15. These are mostly in the Sacramento and San Joaquin river systems, which begin on Mount Shasta and in the Sierra Nevada, respectively, and flow down the Central Valley, through the Delta, and out to the Pacific Ocean.
Preliminary results suggest that Western rivers typically undergo one to three heat waves a year, and not only in the summer. “People don’t always think about it but there can be riverine heat waves in the winter,” Walter says. “In California, we have a whole bunch of species that are adapted to the winter being a good time, with more water in rivers and cooler temperatures.” Temperature spikes during the winter could disrupt life stages of aquatic organisms that are suited to conditions typical of the season.
In contrast with the previous study, which showed that river heat waves have doubled nationwide, the current study shows no such increase across the West. That could change as the world warms, though. “Climate change in general is increasing stream temperatures 2 to 6 ⁰C, especially where we’re losing snowpack,” Null says.
CAN WILDFIRE SMOKE COOL WATER?
The national study showed that the primary driver of extreme temperatures in waterways is heat from the sun. Other causes include factors that diminish streamflow, such as decreases in rainfall and runoff.
A key question in the new study is whether wildfire smoke helps keep rivers from overheating. “One interesting thing that’s different in the West versus the East is the prevalence of wildfires,” Walter says. “We expect the smoke will have a cooling effect because it blocks the sun.” The team is using wildfire smoke data from satellites to see if rivers are indeed cooler on smoky days.
If so, that could inform water management in the West. California, for example, releases cold water from reservoirs when it gets too hot for salmon downstream. But that may not be necessary on smoky days. “If wildfire smoke lessens heat waves in the water, we could save more water in reservoirs,” Null says.
MANAGED AND CULTURAL BURNS
In support of the team’s expectation, a 2018 study found that wildfire smoke can cool streams in the lower Klamath River Basin in the far north of California, where high water temperatures in the summer further jeopardize the endangered spring-run Chinook salmon. The study showed that during six summers with extensive wildfires, smoke cooled the average maximum temperature of the water by 1.3 °C across all 12 study sites—and, at one site, by nearly 4.5 °C.
Study co-author Frank Lake, a U.S. Forest Service ecologist thinks natural fire patterns may have protected salmon and other aquatic species from overwarm water historically. “Prior to modern fire suppression, wildfires burned extensively throughout much of the Western United States, and smoke from these fires may have naturally cooled water temperatures during the summer when temperatures are hottest,” he said in a statement.
In a 2017 preliminary analysis of whether wildfire smoke could cool the Salmon River, Lake and colleagues suggested that prescribed burns and managed wildfire might help keep waterways cool enough for salmon today.
Lake, who has Yurok and Karuk family and is a Karuk descendant, learned traditional ecological knowledge from elders of these Klamath Basin tribes. Now, the 2019 Karuk Climate Adaptation Plan includes his finding that wildfire smoke can cool streams in the basin.
Cultural burns may have benefited salmon “during critical warm periods, creating good conditions for fall migration and spawning,” say the plan authors, who call for “a return to utilization of cultural indicators for prescribed fire, together with the eventual restoration of August ceremonial fires.”
BEAVER DAMS
Other ways to cool rivers and streams include restoring riparian forests, which shade waterways from the sun, as well as floodplains, which connect surface flows with those underground. “Water spreads out and a lot of it seeps into the ground,” Null says. “Then it flows under the surface and wells back up to the river, cooling it in the summer.” Beyond depths of about three feet, ground temperatures are relatively low year-round.
Likewise, beavers can bring stream temperatures down: their dams create ponds that allow water to sink into the cool ground, says Benjamin Dittbrenner, a Northeastern University ecologist who studied the impact of relocated beavers on mountain stream temperatures in Washington state.
Dittbrenner led a 2022 study showing that large beaver complexes, which have five to 10 dams, lowered stream temperatures as much as 6 °C at one of the complexes studied and an average of 2.4 °C across all them. “That’s huge—2.4 °C doesn’t sound like much but it’s really awesome for fish,” he says. Water temperatures above 17 °C can impair coho salmon, for example hindering their ability to hunt, and temperatures above 23 °C can kill them.
Comparing stream temperatures before and after beaver relocation showed that large beaver complexes kept the water cool enough for salmon. Before beaver relocation, streams were warm enough to impair coho during nearly one-third of the summer. After relocation, water temperatures never reached that threshold “even though it was a really hot summer,” Dittbrenner says.
The prevailing theory once held that beavers warm streams, and some studies show they can. Dittbrenner thinks it all comes down to the size of the beaver complex and the depth of the ponds. More dams mean more water in the wetlands beavers create, increasing infiltration into the cool ground. And deeper water is cooler at the bottom due to a process called thermal stratification: cold water, which is more dense, sinks while warm water floats on top.
Beaver ponds can offer salmon relief from the heat. “Beaver dams are porous—they’re made of mud and sticks—and fish can squiggle through,” Dittbrenner says. “Deep ponds can be cool at the bottom even when the surface is hot enough to kill fish.”
Understanding river heat waves in California and elsewhere will help identify waterways where temperatures rise high enough to threaten aquatic life. This in turn will help set priorities for protections that cool water, boosting the climate resilience of salmon and other imperiled species that increasingly rely on cold water refuges to escape the heat.
