How do rivers choose their paths? Why do some rivers form single channels, while others divide into many threads?
The questions may sound simple, but they are plagued by dynamic and complex forces that have long puzzled scientists.
Now, a new study in the journal Science offers some clarity.
“We investigated a longstanding mystery at the intersection of geology, ecology, geography, and engineering: why do some rivers confine flow into a single channel, whereas others split their flow into multiple sub-channels?” said study lead author Austin Chadwick, who conducted the research as a postdoctoral researcher at the University of California, Santa Barbara, in an email to USA TODAY.
It turns out that rivers choose their paths based on erosion – a discovery that could transform flood planning and restoration.
“We found that rivers will develop multiple channels if they erode their banks faster than they deposit sediment on their opposing banks,” Chadwick said. “This causes a channel to widen and divide over time.”
As for what causes this, Chadwick said, “A river with soft, sandy riverbanks and a high rate of flow erodes far faster than a river with hard, muddy riverbanks and a low rate of water flow.”
Why does this matter? As noted in the study, “these channel patterns shape flood risks, erosion hazards, and ecosystem services for more than 3 billion people inhabiting river corridors worldwide.”
Two river categories
Scientists have long divided rivers into single and multichannel categories and usually study the two separately. While neither type dominates, most of the world’s largest rivers are multichanneled. The notable exception is the single-channel Mississippi River, where a lot of river research has occurred.
Most field research has focused on single-threaded rivers, partly because they’re simpler, according to a statement from UCSB. Meanwhile, experimental work has focused on multi-threaded rivers due to the challenges of recreating single-threaded channels in laboratory tank experiments.
“The question of what causes a river to be single-threaded or multi-threaded is pretty much as old as the field of geomorphology,” explained senior author Vamsi Ganti, an associate professor of geography at UCSB. Geomorphology is the scientific study of landforms, their origin, evolution and the processes that shape them.
What real-world implications does the study have?
Chadwick said that “there is growing recognition that many rivers have historically transitioned from multi-channel to single-channel after human interference (for example, damming, diking, sediment mining, clearing and snagging, and agricultural development).”
“Sustainable, nature-based river management aims to restore rivers towards their natural state, but the success of these multibillion-dollar efforts hinges on understanding how much space a river needs and how long it will take to return to its natural state,” he said.
“One important implication for our study is that multi-channel rivers require significantly less space and less time to return to their natural state. This suggests a promising future for nature-based management along historically multi-channel rivers, with reduced restoration costs.”
Ganti further elaborates, noting in an email to USA TODAY that “restoring rivers is essential for protecting biodiversity and ensuring water security, but it’s often a complex and expensive undertaking. Many rivers around the world have been damaged by human activities like dams and channelization.”
“Restoring these rivers is crucial for healthy ecosystems and communities, but it can be expensive and time-consuming. Our research provides a new tool for predicting how long restoration will take and how much space a river needs to recover naturally,” Ganti said.
Does the research have anything to do with how rivers flood?
“Yes,” Chadwick said. “Rivers flood when they are no longer capable of containing the flow within their channel(s). Understanding how rivers forge their channel(s), as we have done in this study, is crucial for anticipating floods in the future.”
Today, the most widely used tools for predicting river floods are based on the assumption of a stable channel width and depth. However, researchers find this assumption breaks down for about half of all rivers (the multichannel ones).
“In the future, I hope we can improve flood forecasts along multi-channel rivers by accounting for how channel widths and depths naturally vary over years to decades,” Chadwick said.
Ganti explained that “when we think about river flooding, we usually focus on water levels rising and overtopping levees. However, rivers are dynamic systems – their banks erode, they shift course and this movement can dramatically change flood patterns.”
“Our study provides a unique global view of these river migration zones. This information is crucial for better predicting future flood risks and developing more effective mitigation strategies,” he said.
This article originally appeared on USA TODAY: A surprising ‘longstanding mystery’ about rivers has a new answer
