Some rivers are soaking up carbon instead of releasing it

For years, scientists thought rivers were mostly releasing carbon dioxide into the air – and that’s no small matter.

Carbon dioxide (CO₂) is one of the main greenhouse gases warming our planet by trapping heat in the atmosphere.

Rivers, even though they’re constantly moving and look clean, were thought to act more like chimneys than vacuums – letting out more carbon than they took in. But that idea may be off, at least for part of the country.

A surprising twist in Western rivers

New research looking at every single river network across the lower 48 states has thrown a wrench into this long-held belief.

It turns out that a lot of rivers in the West – especially in dry areas – might actually be absorbing CO₂ from the atmosphere.

This kind of discovery changes how we think about carbon movement on Earth. If some rivers can pull in carbon instead of releasing it, that shifts how we calculate our carbon budgets and how we might manage emissions in the future.

Not all rivers behave the same

Until recently, most of the data scientists had on rivers came from places like the Northeastern U.S., where forests are thick and rivers wind through leafy, shaded areas.

These rivers get loaded with organic matter like fallen leaves, and they don’t get much sunlight. That combination leads to high respiration (releasing CO₂) and low photosynthesis (absorbing CO₂), so those rivers act like carbon sources.

But that’s just one type of environment. Many rivers in the U.S. – especially in the West – don’t look like that. They’re out in the open, where the sun beats down with no canopy to block it.

There’s also less plant material washing into the water. These conditions give photosynthesis a chance to ramp up and outpace respiration, which can flip a river from a carbon source into a carbon sink.

Why this research is different

The research team was led by Taylor Maavara, a biogeochemist at the Cary Institute of Ecosystem Studies took a much broader view.

Instead of relying on past observations from just a few places, the researchers brought in data from the U.S. Geological Survey and built machine learning models to fill in the gaps.

They looked at hundreds of monitoring sites and trained a computer model to predict how much carbon each river section would absorb or release. The model used information like light, temperature, nutrients, organic matter, and how fast the water was moving.

Then the team scaled it up – across every stream and river network in the contiguous United States.

“Rivers are one of the most uncertain parts of the global carbon cycle,” said Maavara. “So in terms of balancing the global carbon budgets, figuring out where the carbon in rivers is coming from and where it’s going is essential.”

Rivers that absorb more carbon

Once the less-studied Western rivers were added in, a clear pattern emerged. About 25% of river sections in the West take in more carbon than they release. That’s more than double the 11% of river sections in the East that do the same.

It doesn’t mean all rivers are now carbon sinks. Across the country as a whole, they still emit more carbon than they absorb. But the margin isn’t as wide as scientists thought.

That matters, especially when you consider that 65% of Earth’s land is either arid or semi-arid – similar to those carbon-absorbing Western areas.

“Our work suggests rivers that were considered outliers in previous studies may be more common than we thought, especially in these understudied areas,” Maavara said.

Climate change adds new layers

Interestingly, some of the same forces making the climate hotter and drier might be making Western rivers better at soaking up carbon – for now.

Slower-moving water in drier areas means sunlight can reach deeper into rivers. That light boosts photosynthesis, helping rivers absorb more CO₂ from the air. But it’s a narrow window.

If those rivers dry up completely, the effect disappears, and the riverbeds could even become sources of carbon again.

“This study takes us several steps closer to narrowing the big gap in terms of understanding in the carbon cycle, which in turn will help us manage and mitigate the CO₂ in the atmosphere,” said Maavara.

Carbon tracking in rivers

Carbon tracking isn’t just academic. It’s a puzzle we need to solve if we want to understand how to fight climate change.

Rivers might not be the first thing you think about when it comes to climate strategy, but they could become important players in managing carbon over time – especially in dry parts of the world.

This research doesn’t close the book on how rivers behave, but it opens a big new chapter. Now scientists have better tools and a clearer map to keep exploring.

The full study was published in the journal Science.

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