What causes rivers to suddenly change course?

Displacement is in the nature of a river. But when a river breaks from its channel and carves a new path across the landscape, devastating floods can descend on communities with little or no warning.

For decades, researchers have struggled to explain exactly how river channels prepare for such sudden diversions or avulsions. A study published on September 18 in Nature may have finally settled the debate, showing how two factors work together to orchestrate a river’s course change. Based on their findings, the researchers also developed a promising algorithm that can predict the new path of a fallen river.

“These are monumental floods, civilization-changing floods in some cases,” says sedimentologist Douglas Edmonds of Indiana University in Bloomington. In 2010, dams on the Indus River in Pakistan contributed to floods that forced approximately 20 million people from their homes. However, flood risk models remain unable to predict where rivers will be redirected, Edmonds says. “It’s really an invisible flood hazard.”

Avulsions require a setup and a trigger – an overburdened camel’s back and a last straw (SN: 28.6.24). “The trigger could be a flood, an earthquake, it could be a blockage in a river,” says Edmonds. Configuration refers to how sediment deposition has prepared a river to divert—and is the root cause of diversion, Edmonds says. “Rivers flood all the time, but they don’t burst all the time.”

The new study focused on determining the structure, for which there were two competing hypotheses. Avulsions were thought to occur when a river rises too high, or sediment deposition raises the water level of a river above the surrounding land. The other asserted that avulsions occur when there is a slope advantage, or when the slope of a new, potential path becomes steeper than that of the current river path.

Edmonds and his colleagues began by using satellite data to investigate approximately 170 avulsions, noting how far downstream rivers tend to divert. They found that avulsions were roughly three times more common near river mouths and mountain fronts than in the middle.

Focusing on 58 river channels for which high-resolution topographic data were available, the researchers measured the advantage of elevation and slope before subsidence. They found that elevation best explained avulsions near mountains, while slope advantage best explained those near estuaries and deltas.

There’s so much sediment coming down from the mountains that the rivers just collect it until they get too high and overflow, Edmonds says. Meanwhile in deltas, there is a lot of cohesive mud that forms very steep natural sheets around deep channels, and avulsions need a big slope advantage to start cutting the slope, he adds.

These two factors – slope advantage and overhang – work together in an opposite way, the researchers found. The more congested a river becomes, the less advantageous the slope is for it to leave, and vice versa. “It’s the first time anyone has been able to show this with data,” says Penn State geologist Elizabeth Hajek, who was not involved in the study.

Avulsions occurred when the mathematical product of the two factors exceeded a threshold value, the researchers found. As long as accurate topographic measurements of a river’s channel are available, which is more likely for larger rivers and in clear-sky locations, you can probably use that threshold metric to identify where avulsions are likely to occur, says geomorphologist Vamsi Ganti of the University. of California, Santa Barbara, who was also not involved in the study.

The researchers developed a computer algorithm that outlined where on a map an inverted river might go, taking into account the slope of the terrain and the speed of the river. When tasked with predicting the paths of 10 past avulsions, the algorithm correctly captured the path of each one. “It’s a really good tool,” says Hajek. “It can be really, really useful for identifying areas of concern.”

The plan is to develop avulsion risk maps for the globe or vulnerable regions, Edmonds says. “Now that we have this metric, we can measure it in rivers all over the world.”