Heatwaves in US rivers increasing up to four times faster than air heatwaves

UNIVERSITY PARK, Pa. — As the frequency and intensity of heatwaves increase across the U.S., a similar but more striking phenomenon is occurring in American rivers. Analysis of data from nearly 1,500 sites in the contiguous United States between 1980 and 2022 revealed that heatwaves in rivers are accelerating faster than and lasting nearly twice as long air heatwaves, according to a new study by researchers at Penn State.  

“Rivers are often thought of as safe and cool havens protected from extreme temperatures,” said Li Li, corresponding author and Barry and Shirley Isett Professor of Environmental Engineering in Penn State’s Department of Civil and Environmental Engineering. “Our study shows, for the first time, that rivers are experiencing a more rapid increase in frequency, duration and intensity of heatwaves than air, and are increasing at about two to four times the rate of air heatwaves.” 

The research, published in the journal PNAS, found that periods of abnormally high riverine water temperatures are a threat to aquatic ecosystems, water quality and America's food supply and energy stability. 

“Our findings show that riverine heatwaves are increasing faster than air heatwaves, a trend likely happening worldwide,” said Kayalvizhi Sadayappan, lead author and postdoctoral scholar in the Department of Civil and Environmental Engineering at Penn State. “This raises the risk of rivers experiencing both extreme heat and low water flows at the same time, which can cause conditions that can lower oxygen levels, stress aquatic life and even trigger large-scale fish die-offs.” 

Rising river temperatures can put pressure on several aquatic species, many of which are cold-blooded, especially cold-water fish such as salmon and trout, Sadayappan said. The study found that since 1980, rivers across the contiguous United States have experienced an average increase of 11.6 days per year when water temperatures rose above 15 degrees Celsius (C), or 59 degrees Fahrenheit (F) — a temperature threshold that can stress many species. These warm-water days have climbed the fastest in the Northeast, the Rocky Mountains and the Appalachian region, with significant increases observed at 82% of the sites studied. 

The researchers also tracked more extreme conditions, defined as days when river temperatures exceeded 20 C, or 68 F. They found that critical thermal stress days are rising most quickly in the Southern and Appalachian regions, with significant increases recorded at 74% of sites. 

By contrast, rivers in the Midwest showed the slowest rise in both categories, adding only about five extra days per year above 15 C and less than one day per year above 20 C. 

In addition to harming water quality and threatening aquatic life, riverine heatwaves impact livelihoods and cultural traditions that revolve around these ecosystems, especially for Indigenous communities, Sadayappan said. They also raise drinking water treatment costs, limit recreation and farming, and disrupt energy production by causing shutdowns and wasting energy. 

According to Li, air heatwaves generally receive more attention because they directly impact humans, while riverine heatwaves often go unnoticed unless they cause consequences that are more visible, such as a massive fish die-offs. 

While heatwaves in lakes, oceans and large rivers are well-documented through satellite data, approximately three out of four rivers globally escape satellite observation, she explained. In addition, data from decades of stream sensor measurements are fragmented and inconsistent. To address this gap, the researchers developed a model where computers learn patterns and make predictions from large amounts of data to reconstruct consistent and continuous daily water temperatures. 

“Deep learning enabled us to identify riverine heatwave events and quantify their characteristics and trends over four decades,” Sadayappan said. “Without the deep learning approach and the reconstructed data, we wouldn’t have recognized that riverine heatwave events have been increasing more rapidly than air heatwaves.” 

This work highlights the urgent need to monitor and mitigate the accelerating rise of riverine heatwaves, Sadayappan said. 

The study suggested that climate change is the primary force behind increasing trends of riverine heatwaves, as rising air temperatures strongly influence river conditions. Precipitation patterns, especially winter snow, also play a key role. In mountain regions like the Rockies, shrinking snowpacks mean less cool meltwater to buffer rivers, leaving them more vulnerable to heatwaves. 

Human activities, such as dams and agriculture, play a secondary role in shaping how and where rivers are most vulnerable to these impacts, Sadayappan added. 

“Agriculture has been mitigating riverine heatwaves via cooler air and water during irrigation, as indicated by declining trends in the frequency, duration and intensity of riverine heatwaves in crop-cultivated areas,” she said. “On the other hand, dams have been accelerating trends in riverine heatwaves. In particular, large dams have been contributing towards elongating riverine heatwaves.” 

Sadayappan said that rivers in landscapes with more human activity, such as cities, generally experience more frequent, longer and more intense riverine heatwaves, but rivers in undeveloped landscapes are rapidly catching up. 

“This information can provide warning signals and support adaptive management during riverine heatwaves,” Sadayappan said. “More importantly, it can inform long-term mitigation efforts to restore vegetation along riverbanks to provide shade, improve dam management, expand green infrastructure and reduce water diverted from rivers.” 

The team said they hope their work could lead to policies and incentives that are essential to protect vulnerable river ecosystems against heatwaves and the people who depend on them. 

This study was funded by the U.S. National Science Foundation, Penn State’s Institute of Energy and the Environment and the Barry and Shirley Isett Professorship at Penn State.