US rivers are getting hot.
A new analysis of nearly 1,500 streams over more than 40 years finds that the frequency, intensity and duration of heat waves in streams across the country are increasing, posing a threat for many species adapted to lower temperatures.
“The trend of river heat waves is actually increasing faster than air heat waves. And that's very surprising,” said Lee Li, a professor of environmental engineering at Pennsylvania State University and an author of the study.
new analysisThe study, published in the peer-reviewed journal Proceedings of the National Academy of Sciences, is the first in-depth study of riverine heatwaves, which are defined as five consecutive days of high temperatures compared to the seasonal average.
The authors found that human-caused climate change is the main driver of this trend, as snowpack dwindles and rivers flow more slowly.
Other human factors also influence this trend. Dams slow down the flow of water downstream. Buildings and sidewalks absorb heat, which warms the air and then the water.
Scientists expect species that rely on cold water, including salmon and trout, to struggle as rivers warm. Lee said it will be important to document the fish kills to see if and what trends are emerging.
Heat reduces the amount of dissolved oxygen in water. Meanwhile, cold-water species often experience increased metabolism in warmer waters, meaning there is less oxygen.
The findings could help understand how to manage watersheds and whether changes in irrigation or dam releases could help species survive hot weather.
“How we use water, how we manage these resources, can influence heat waves,” said Jonathan Walter, a research scientist at the Center for Watershed Sciences at the University of California, Davis, who was not involved in the study. “Assuming there was enough water in the reservoir, we could release the water in a way that would lower the temperature downstream somewhat and prevent those high and stressful temperatures.”
Heat waves have been widely studied in oceans and lakes, but until now researchers had only limited information about how warmer temperatures affect streams.
Satellite data is fueling research into heat in oceans and lakes. Studying river temperatures is more difficult because it relies on water gauge sensors that often cycle in and out, leaving gaps in data that are difficult to analyze.
For the new study, researchers collected data from 1,471 USGS river monitoring sites from 1980 to 2022.
The data allowed the researchers to assess changes in flows across the country. They found that riverine heatwaves occurred more frequently in 2022 than in 1980, with an average of 1.8 additional heatwaves occurring per year. Heat waves were also more intense, with temperatures during the 2022 heat wave averaging about 0.8 degrees Fahrenheit higher than in 1980. The heat waves lasted more than three days longer than in the past.
Lee said the extra heat causes an average of about 12 days of heat stress in species that need cold water. The study authors used a temperature of 59 degrees Fahrenheit as the threshold for heat stress because some species, such as bull trout, struggle to survive at this mark.
“When we think about water, there's a lot more emphasis on quantity because we can see it,” Lee said. “We hope this type of research will increase public awareness of the water quality problem associated with climate change.”
