Western Rivers in the United States Could Be Powerful Unexpected Allies in the Fight Against Climate Change

Recent research published in Science is reshaping how we think about rivers and their role in the global carbon cycle. For decades, scientists generally believed that most rivers release more carbon dioxide into the atmosphere than they absorb. That idea has been so widely held that it became almost an assumption in climate science. But a massive new analysis led by aquatic biogeochemist Taylor Maavara from the Cary Institute of Ecosystem Studies challenges this old picture. It reveals that many western U.S. rivers, especially those flowing through arid and semi-arid landscapes, may actually act as carbon sinks, pulling in more carbon dioxide than they emit.

This finding matters because it forces the scientific community to reconsider how we account for the carbon cycle in freshwater systems — and it highlights the importance of regions that historically received far less research attention.

A New Look at River Metabolism Across the Entire U.S.

The study is the largest analysis of river metabolism ever conducted in the contiguous United States, covering every river network across the country. It focuses on understanding the interplay between two key processes:

Photosynthesis — when aquatic plants and algae absorb carbon dioxide
Respiration — when plants, animals, and microbes release carbon dioxide back into the air

The balance between these two processes determines whether a river reach is a carbon sink or a carbon source.

The problem is that until now, most long-term data on river metabolism came from temperate, forest-covered regions — especially in the Northeastern U.S. These rivers are usually shaded, have lots of leaf litter entering the water, and tend to emit more carbon dioxide than they absorb. Because so much research centered on these rivers, scientists assumed the same patterns applied everywhere.

Maavara and her team wanted a more complete picture.

How the Study Was Done

To capture national-scale trends, the research team used a combination of direct measurements and machine-learning modeling.

Here’s how they approached it:

Compiled U.S. Geological Survey data from hundreds of monitoring sites measuring oxygen fluctuations, which reveal photosynthesis and respiration patterns.
Identified key environmental factors, including light levels, temperature, organic matter, nutrients, and flow rates.
Used machine learning to upscale these data and estimate monthly and annual river metabolism values for every river reach in the United States, even in places where direct measurements never existed.

This approach allowed them to include long-overlooked rivers in deserts, shrublands, and arid basins, which turned out to behave very differently from their forested eastern counterparts.

What Makes Western Rivers Different?

The study uncovered a surprising pattern: arid western rivers often lean toward net photosynthesis, meaning they absorb carbon dioxide rather than releasing it.

Several factors explain this shift:

More sunlight
With fewer trees and less canopy cover, sunlight reaches the water easily, boosting photosynthesis.
Lower organic carbon inputs
In dry landscapes, less leaf litter and soil organic matter wash into streams. That means there’s less fuel for respiration, which lowers carbon emissions.
Warmer temperatures and slower flows
Climate-driven warming and reduced precipitation slow river flow, giving more time for sunlight to penetrate and drive photosynthesis.

Because of these conditions, about 25% of western river reaches function as carbon sinks, compared to only 11% in the East.

That number is especially significant because arid and semi-arid lands cover nearly 65% of Earth’s land surface. If similar river systems elsewhere behave like those in the American West, global models may be underestimating how much carbon rivers can absorb.

Rivers Still Emit More Carbon Overall — But Far Less Than Previously Thought

It’s important to clarify that when all U.S. rivers are combined, they still emit more carbon dioxide than they absorb. However, the new analysis shows that the deficit is far smaller than earlier estimates, which mainly relied on forested river data.

This more balanced picture also suggests that many rivers thought to be “outliers” — like the bright, sun-drenched western streams — are actually far more common globally than old datasets implied.

Climate Change May Be Temporarily Boosting River Carbon Uptake

One intriguing part of the study is how climate change interacts with river metabolism.

In the western U.S., climate change is leading to:

Hotter temperatures
Less precipitation
Reduced river flow rates

Paradoxically, these changes can increase carbon uptake in flowing river segments by allowing more sunlight and lengthening water residence time. But the benefit is fragile. If rivers dry up completely — which is happening more often in parts of the Southwest — they lose their ability to act as carbon sinks and may become carbon sources instead.

So while western rivers are currently gaining carbon-absorbing potential, that trend could reverse sharply if climate conditions continue to worsen.

Why This Research Matters for Climate Science

This study narrows one of the large uncertainties in the global carbon budget. Rivers have been notoriously difficult to account for because their metabolic processes vary widely.

Key takeaways for climate science include:

Carbon models need updating to reflect the diversity of river ecosystems.
Arid rivers may play an unexpectedly big role in regulating atmospheric carbon.
Eastern forested rivers are not universal examples, despite decades of dominant research bias.
Understanding river metabolism is essential for accurate climate mitigation planning.

What This Means for Conservation and River Management

If arid-land rivers can naturally absorb carbon, they represent an ecological service that hasn’t been fully recognized. Properly managed waterways could help offset greenhouse gas emissions — not as a silver bullet, but as a meaningful contribution.

However, this only works if the rivers keep flowing.

Drying rivers lose their metabolic balance. That places new emphasis on:

Maintaining minimum flow levels
Protecting riparian zones
Managing water withdrawals
Monitoring drought impacts

Water managers and policymakers may increasingly consider carbon-cycle impacts alongside biodiversity and supply-related decisions.

Additional Context: How River Metabolism Works

To give readers more background, here are the two essential processes driving river carbon dynamics:

Photosynthesis

Aquatic plants, algae, and cyanobacteria absorb carbon dioxide and use sunlight to produce oxygen and organic matter. High light availability and slower water movement often increase photosynthetic activity.

Respiration

Plants, animals, and microbes consume oxygen and release carbon dioxide. Respiration increases when large amounts of organic matter (like leaves or soil carbon) enter the water.

The balance between these two determines whether a river reach acts as a net carbon sink or net carbon source.

Western rivers tilt toward photosynthesis because of high sunlight and low organic input, while eastern forested rivers tilt toward respiration due to dense canopy cover and heavy leaf litter.

Looking Ahead

This research highlights how much we still have to learn about river ecosystems. The authors note that uncertainties remain, especially regarding intermittent streams, long periods of drought, and future climate scenarios.

But the study is a major step toward refining our understanding of freshwater carbon cycling — and it reveals that some of Earth’s driest landscapes may hold surprising climate benefits.

Research Reference:
https://www.science.org/doi/10.1126/science.adu9843