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As the 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 heat waves in rivers are accelerating faster than and lasting nearly twice as long as air heat waves, 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 heat waves than air, and are increasing at about two to four times the rate of air heat waves."

The research, published in the journal , found that periods of abnormally high riverine water temperatures are a threat to aquatic ecosystems, and .

"Our findings show that riverine heat waves are increasing faster than air heat waves, 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 heat waves 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 heat waves generally receive more attention because they directly impact humans, while riverine heat waves often go unnoticed unless they cause consequences that are more visible, such as massive fish die-offs.

While heat waves 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 heat wave 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 heat wave events have been increasing more rapidly than air heat waves."

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

The study suggested that climate change is the primary force behind increasing trends of riverine heat waves, 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 heat waves.

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 heat waves via cooler air and water during irrigation, as indicated by declining trends in the frequency, duration and intensity of riverine heat waves in crop-cultivated areas," she said.

"On the other hand, dams have been accelerating trends in riverine heat waves. In particular, large dams have been contributing towards elongating riverine heat waves."

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

"This information can provide warning signals and support adaptive management during riverine heat waves," 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 heat waves and the people who depend on them.