America’s rivers are heating up faster than the air

As summer temperatures rise, rivers across the United States are facing a different kind of heatwave – one that’s often overlooked.

Stay indoors. Drink water. Check on your neighbors. These are the warnings we hear when the air gets too hot. But beneath the surface, something more troubling is heating up.

Over the last 40 years, rivers across the United States have been heating up faster and staying hot longer than the air above them.

These “riverine heatwaves” aren’t just a weird twist of climate change. They’re reshaping how rivers work and putting serious stress on life that depends on them – including ours.

Overlooked heatwaves in our rivers

Rivers aren’t usually seen as places where heatwaves happen. They’re often thought of as cool, flowing refuges – not places you’d associate with extreme heat. But that’s changing, fast.

From 1980 to 2022, researchers looked at nearly 1,500 sites across the lower 48 states. What they found was alarming: riverine heatwaves are increasing at two to four times the rate of air heatwaves. And when they hit, they last almost twice as long.

“Rivers are often thought of as safe and cool havens protected from extreme temperatures,” said Li Li, a professor 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.”

What hotter rivers mean for us

River heatwaves are hitting aquatic life where it hurts. Cold-water fish like trout and salmon don’t have much wiggle room – their body temperature depends on the water around them. When rivers warm up too much, these fish struggle to function, let alone survive.

The researchers found that, on average, rivers in the U.S. now see about 11 more days each year when temperatures climb past 59°F – a point where many species start to feel the stress. The biggest jumps showed up in the Northeast, the Rocky Mountains, and the Appalachians.

Things get even more serious when river temperatures pass 68°F. That’s when heat stress becomes critical. Those extra-hot days are increasing the fastest in the South and Appalachian regions, showing up at nearly three-quarters of the sites researchers looked at.

“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,” said Kayalvizhi Sadayappan, the study’s lead author.

Not all rivers are heating the same way

There’s some variation in how fast things are heating up. The Midwest showed the slowest increases – about five more warm-water days per year and less than one extra day per year in the critical stress range.

Still, most rivers are following the same trend: warming more and more, especially in places where people are altering the landscape.

According to the study, rivers in urban or heavily developed areas already experience more frequent, longer, and more intense heatwaves. But rivers in less-developed regions are catching up quickly.

What is driving riverine heatwaves?

The main driver is climate change. As air temperatures rise, so do river temperatures. But that’s not the whole story. Other factors like snowmelt, rainfall, dams, and farming also shape how rivers react to heat.

In mountainous areas like the Rockies, snowpack used to act like a cooling system. But with warmer winters and shrinking snow levels, less cold meltwater flows into rivers. That leaves them more exposed to heating.

Human activity can make things worse – or better. According to the research, dams tend to make riverine heatwaves worse, especially large ones that stretch out the length of hot water events. In some areas, farming has played a role in keeping river temperatures down.

“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,” said Sadayappan.

“On the other hand, dams have been accelerating trends in riverine heatwaves. In particular, large dams have been contributing towards elongating riverine heatwaves.”

Communities feel the heat too

River heatwaves don’t just hit wildlife. They mess with everyday life. When water gets too warm, drinking water treatment becomes more expensive. Recreational activities get canceled. Farms suffer. Power plants that rely on river water have to shut down, wasting energy and driving up costs.

The impacts are especially serious for Indigenous communities who rely on rivers for fishing, food, and cultural traditions. Warm rivers mean disrupted traditions and endangered resources.

And because rivers usually don’t get attention unless something dramatic happens – like a mass fish die-off – these changes often go unnoticed until it’s too late.

Decades of river temperature data

Tracking river heatwaves isn’t easy. While satellites can monitor heat in oceans and large lakes, about three out of every four rivers worldwide aren’t picked up by satellite sensors. And the data collected on the ground is often spotty.

To fill in the gaps, the researchers built a deep learning model. It used artificial intelligence to sort through decades of river temperature data and spot patterns in where and when heatwaves happened.

“Deep learning enabled us to identify riverine heatwave events and quantify their characteristics and trends over four decades,” said Sadayappan.

“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.”

The warning signs are here

The research points to a need for more attention, monitoring, and action. Rivers are heating fast, and the consequences are stacking 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.”

Solutions will require policy changes and smart incentives that help protect river systems – and the people who depend on them.

The full study was published in the journal Proceedings of the National Academy of Sciences.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–