Why rivers split: A century-old mystery has finally been solved

Rivers shape life on Earth. They deliver water, sediment, and nutrients from mountains to oceans, cutting paths that feed ecosystems and human communities alike.

For over a century, scientists have tried to understand why some rivers form a single, meandering channel, while other rivers split into multiple, crisscrossing streams. New research finally offers an answer.

What causes a river to split?

In a study by geographers at the University of California, Santa Barbara, scientists analyzed 36 years of satellite data from 84 rivers around the world. The goal was to determine what causes a river to split into many channels rather than stick to one.

Study lead author Austin Chadwick conducted the investigation as a postdoctoral researcher at UCSB.

“We found that rivers will develop multiple channels if they erode their banks faster than they deposit sediment on their opposing banks. This causes a channel to widen and divide over time,” said Chadwick.

The study settles a long-standing debate in Earth science and provides new insight for understanding flood risks and how to restore waterways to their natural states.

Studying two types of rivers

Rivers are typically sorted into two categories: single-threaded and multi-threaded. While both are common, most of the world’s largest rivers have multiple channels.

A notable exception is the Mississippi River in the U.S., which has been heavily studied and remains single-threaded.

Historically, single-threaded rivers got more attention from field researchers because they are easier to study. Lab researchers, on the other hand, often focused on multi-threaded rivers since they were easier to recreate in tank experiments.

The idea for this study started in a lab tank at the University of Minnesota’s St. Anthony Falls Laboratory. While observing multi-threaded rivers, Chadwick noticed something strange. The river channels kept widening and splitting, and erosion outpaced deposition.

“I was banging my head on the wall because I kept measuring more erosion than deposition. And that was not what we’re taught in school,” said Chadwick. “That led me to read some old books from the Army Corps and other sources about examples where there’s more bank erosion than deposition.”

This curiosity led to a broader question: was the odd river behavior just a fluke in the lab – or did it happen in nature too?

“You generate a hypothesis in a laboratory setting and then you’re able to test it in nature,” said co-author Evan Greenberg, a former doctoral student at UCSB.

How rivers choose their path

To test their hypothesis, the team turned to satellite imagery, specifically Landsat data available through Google Earth Engine. They studied erosion and deposition patterns across dozens of rivers using a technique called particle image velocimetry.

Originally designed to track particles in lab fluids, they adapted it to track riverbanks in satellite photos.

The results were clear. Single-threaded rivers show a balance between erosion and deposition. This balance allows them to stay narrow and meander across the landscape.

Multi-threaded systems behave differently. In these rivers, erosion wins out. Banks erode faster than sediment can build up on the opposite side. The result? The channel keeps widening until it eventually splits.

“It is not like multi-threaded rivers are gaining water on average. They are still conveying the same amount of water through time, but they are doing that by constantly shuffling the size of the individual threads,” explained senior author Vamsi Ganti, an associate professor of geography at UCSB.

New insights into why rivers split

The key difference isn’t how much water flows – it’s how that flow reshapes the banks and moves sediment.

Most of the sediment in multi-threaded rivers ends up on the riverbed, not the banks. This eventually forms the islands and bars that separate each thread.

There were a few exceptions. For example, Brazil’s São Francisco didn’t follow the pattern, but the team traced that back to external changes. The river has been shrinking due to upstream dams and irrigation, pulling it out of its usual behavior.

“The question of what causes a river to be single-threaded or multi-threaded is pretty much as old as the field of geomorphology,” said Ganti.

Now, thanks to this work, there’s a clearer explanation: it comes down to the balance between erosion and deposition.

Environmental conditions – like slope, flow, sediment type, and bank strength – still matter. But they influence the river by tipping that balance one way or the other.

Rivers cut off from their floodplains

In the 20th century, many rivers were confined to narrow channels to make space for cities and agriculture. While that reduced flooding risks in some areas, it came with a cost.

Cutting rivers off from their floodplains disrupts ecosystems. It also raises the riverbed, making floods worse when they happen.

“Consider Hurricane Katrina,” Chadwick said. “When the levee broke, there was widespread flooding in part because the floodplain had been cut off from the Mississippi for so long that it had sunk relative to the river, allowing the floodwaters to pond there.”

Today, there’s a push to reconnect waterways with their floodplains. But to do that effectively, planners need to know how much space is required – and how long restoration will take.

This study offers a solution. The team developed a formula that calculates corridor width and recovery time for a channel returning to its natural state. It accounts for how quickly a river can abandon an old channel and whether it tends to be single- or multi-threaded.

Rethinking restoration projects

The researchers found that single-threaded rivers need about ten times more space and time to re-establish themselves than multi-threaded ones, assuming equal stream power. That changes the equation for restoration – and may make some projects more feasible.

Photos from the 1930s show the Los Angeles River once had multiple channels before it was confined to concrete.

Many rivers we now think of as single-threaded may have originally been multi-threaded. That could mean restoration won’t require as much effort – or money – as previously believed.

“A project currently considered prohibitively large or expensive may actually be affordable if a river was misclassified,” Chadwick explained.

Following the flow into the future

Ganti’s team is now studying how river behaviors change over time – whether these speed up, slow down, or change how many threads they have. These trends could reflect the effects of climate change and human development.

Chadwick, now continuing his research at Columbia University’s Lamont-Doherty Earth Observatory, is still exploring why erosion dominates and what drives the variety among multi-threaded systems.

Meanwhile, Greenberg is using satellite data at NASA’s Jet Propulsion Laboratory to study how sediment moves through rivers. He’s also examining how dams influence river shapes over decades.

Rivers do more than flow. They feed farms, shape cities, and carry goods across continents. But they also flood, shift, and surprise us.

This new research brings us closer to understanding how these forces shape our world – and how we can better live alongside them.

The full study was published in the journal Science.

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