What happens when rivers jump their banks?

Rivers have been a steady source of life for many civilizations growing on their banks over thousands of years. Water for drinking, irrigation and sanitation have always been essential. 

However, there is a darker side to rivers. When they flood their banks, these powerful bodies of water can destroy crops, bring down buildings, and cause deaths. 

Modern engineers strive to control rivers, but they can be unpredictable. Sometimes floods can even force a river to take an entirely new course, something called a river avulsion. 

Now, a team of researchers from UC Santa Barbara has created the first list of river avulsions. The study finishes off a decade of work by the research group, and will significantly increase our understanding of avulsions. 

 “This dataset provides the first unambiguous test of the theory, which demonstrates that there are three distinct regimes of avulsions on fans and deltas,” said study co-author Professor Vamsi Ganti of UC Santa Barbara. “This is a long way from where we started,” he added. “A decade ago, avulsions were thought to be these chaotic and stochastic events that were not very predictable.”

Because of the rarity of avulsions, they have been difficult to study and little has been known about them. Typically, a river would need to be studied for a long period of time for avulsions to be observed. Satellite photos are an answer to this problem – allowing researchers to instead monitor the whole planet looking for avulsions. 

“Although avulsions are very rare, when you’re looking at practically every single delta on Earth, you’re going to get lucky on a few of them,” said study co-author Austin Chadwick, a postdoctoral scholar at University of Minnesota. 

The team got lucky 113 times while combing the satellite data from 1973 to 2020 and historical maps. “Instead of having these few, deeply studied sites, we now have a representative sample of everywhere on Earth for the last 50 years,” said Chadwick. 

The researchers found three types of avulsions. In the first, the river moved to a flatter landscape where it fanned out. In the second, the river avulsion was limited to a backwater area. Thirdly, the avulsion carried sediments quite far upstream. 

Understanding avulsions can be important for human planning, especially as climate change continues to intensify. The scientists hope that this study is the start of a deeper understanding of avulsions and how they will affect us. 

The study is published in the journal Science.

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By Zach Fitzner, Earth.com Staff Writer