New models detail how major rivers will respond to changing environmental conditions

From the Nile to the Mississippi and from the Amazon to the Yangzi, human civilization is inextricably linked to the great rivers along which our societies developed. But rivers are mutable, and the benefits they provide can quickly become disasters when these waterways change course.

Scientists are working to understand how environmental changes alter river dynamics. A new study in the procedures of the National Academy of Sciences Co-author of Vomsi Ganti, a geomorphologist at the University of California at Santa Barbara, described the factors that determine how often rivers jump off the course or avulse, and the effects this will have on river deltas. The results promise to help scientists and planners prepare for a future of rising sea levels and change in land use.

The deltas counteract rising sea levels by accumulating sediment, which occurs primarily near a river channel. Occasionally, the river will change course through an avulsion and begin to build the delta elsewhere. “So avulsions are the way the river spreads its sediment across the landscape,” said first author Austin Chadwick, a postdoctoral researcher at the University of Minnesota.

“The questions we are asking are how often rivers naturally change their course,” he continued, “and how that will change with climate change and human interference.”

Unfortunately, there has previously been no consensus on how rivers responded to climate change. Some scientists thought that avulsion rates would increase as sea level rises, while others predicted that they would decrease. “There was simply no unifying theory to explain how the river’s avulsion frequency depends on sea level,” said Ganti.

To clarify the situation, Ganti, Chadwick and their co-author Michael Lamb of Caltech combined observations from geological and historical records with a mathematical model of river dynamics. By focusing on this specific topic, his goal was finally to get definitive answers and useful predictions.

Large rivers tend to flatten and slow down as they approach the ocean. After a certain point, conditions downstream from sea level begin to influence the behavior of the river in what scientists call backwater hydrodynamics. “This is a dynamic area where deposition and erosion occurs in coastal rivers,” explained Ganti.

In a previous article, the team had shown that avulsions occur within this backwater region, which can extend far inland. For example, the backwater area of ​​the Mississippi River reaches 500 miles from the shoreline. Deeper, flatter rivers like the Mississippi, which have larger backwater regions, therefore have larger deltas.

The researchers’ goal with this study was to apply their newly discovered understanding of the impact of backwater hydrodynamics to learn about the frequency of avulsions.

Using the model and comparing their results with field data, the team discovered that there are three ways that deltas can respond to rising sea levels, depending on the balance between the rate of change in sea level and the sediment supplied. by the river.

The first: when a river has a lot of sediment and the sea level rise is relatively slow. According to the model, these rivers are resistant to rising sea levels and their avulsion rates remain stable. China’s Yellow River is an example.

The second case occurs when a river has less sediment or the sea level rises faster. In this scenario, avulsions become more frequent. The rising ocean promotes sedimentation, and once a channel is filled to a certain depth, the river will jump its course.

And representing the extreme, in which the rise in sea level exceeds the capacity of a river to deposit sediment, is the third case. As the ocean infiltrates the delta, the river will reach its maximum avulsion rate and the entire system will begin to migrate inland. Scientists did not know about this case before, and the discovery of the three regimes together explains previous inconsistencies in the scientific literature.

The researchers entered observations and data into their model to see if several river deltas would behave differently under predicted weather conditions. “The answer is yes, for most of them,” said Chadwick. “Many rivers will experience more frequent avulsions and some rivers will also have inland avulsions.”

River avulsions have huge social implications, with the potential to cause economic and civil unrest. Archaeologists believe that a change of course of the Indus River in western India directly contributed to the decline of the Bronze Age Harappan civilization. More recently, the avulsions led to the flooding of the Yellow River in 1877 and the floods of China in 1931, two of the deadliest natural disasters in modern history.

An avulsion could have dire consequences for rivers like the Mississippi, where a system called the Old River Control Structure has prevented the river from jumping since 1963. If the backwater region migrates inland, the river could change the course upstream of the installation and avoid it altogether. Millions of gallons of water per minute would pass through previously dry land, while the downstream part of the canal would dry up completely.

The authors have made their model available and accessible to anyone who wants to use it. They were even able to reduce several formulas into a single equation by implementing some basic assumptions about the conditions and dynamics of the river.

“Groups like the Army Corps of Engineers and the Department of the Interior can use this tool to apply to any delta,” Chadwick said. “And hopefully it will help inform our decisions in these places as we tackle climate change.”