World’s first river avulsion dataset validates a decade of modeling and theory | UCSB
Humans have always had a complex relationship with rivers, which have both aided and threatened civilizations throughout history. Just remember Osiris, the ancient Egyptian god of death and rebirth, who was inextricably linked with the annual flooding of the Nile.
Large floods sometimes force a river to change course and carve a new path through the landscape, in rare and catastrophic events known as river avulsions. These events can wipe out entire cities along major waterways, but they also create the fertile deltas that have nurtured many societies.
A team of scientists led by UC Santa Barbara has just published the world’s first compilation of river avulsions in the journal Science(link is external). The study corroborates about a decade of theoretical and experimental work by the group, which fleshed out avulsions of what had been an understudied curiosity.
“This dataset provides the first unambiguous test of the theory, which demonstrates that there are three distinct avulsion regimes on ventilators and deltas,” said co-author Vamsi Ganti(link is external), Professor Assistant in the Department of Geography at UCSB.
“It’s a long way from where we started,” he added. “Ten years ago it was thought that avulsions were these chaotic, stochastic events that weren’t very predictable.”
The rarity and elusiveness of avulsions had mostly kept researchers in the dark. Prior to this article, scientists had only observed a handful. From these few case studies, they began to develop a theory of where avulsions occur using experiments and computer models.
A river may only change course once a decade, once a century, or even less. Scientists must therefore monitor a river for a long time to obtain useful data. However, satellite imagery allowed the team to trade off large stretches of time for large expanses of space.
“Although avulsions are very rare, when you look at virtually every delta on Earth, you’ll be lucky on a few of them,” said co-author Austin Chadwick, a postdoctoral researcher at the University. of Minnesota. The team got lucky 113 times by combining satellite data from 1973 to 2020 and historical maps.
“Instead of having these few deeply studied sites, we now have a representative sample from all over Earth over the past 50 years,” Chadwick said.
Avulsion behavior fell into three regimes. The team found 33 instances where rivers changed course as they moved from steep, confined channels to flatter topography. These fans often occurred at the foot of mountains where a river exits the canyon to unconfined valleys or open oceans. Avulsions of this type required at least a three-fold break in the river valley slope, with a median of 6.5.
Eddy-scale delta avulsions
In the second regime, the avulsions were limited to the eddy zone of a river. “In simple terms, the backwater zone is the part of the river that flows differently due to the effects of sea level at the downstream end,” Chadwick explained. This region can extend surprisingly far inland: about 300 miles for the Mississippi River, for example.
This second group covered 50 of the avulsions in the dataset. These avulsions are found on low-slope deltas along some of the world’s largest waterways, such as the Orinoco, Yellow, Nile, and Mississippi rivers. Most of the group’s previous case studies fall into this category.
Delta avulsions of extreme sediment loads
The final diet included the remaining 30 delta avulsions. In these rivers, intense flooding and sediment transport have pushed avulsions far upstream. Really far.
“The third regime had an avulsion length that was, on average, 14 times the eddy length of the river,” Ganti said. This could extend to over 50 times the length of the backwater in some of the most extreme examples.
The team first documented this behavior in a 2020 paper on Malagasy rivers. “But now we know it’s not just a weird case that we saw in Madagascar,” said Chadwick, who will join Ganti’s Surface Processes group at UCSB in the summer of 2022.
“It’s a third diet of avulsions,” Ganti added. In fact, it accounts for 40% of delta avulsions in the global dataset.
Avulsions have everything to do with sediment transport. They occur when and where a river fills with sediment. This chokes the channel to the point where it jumps elsewhere. On fans, this happens when the slope changes: the flow slows down and the sediment that the stream was carrying settles into the river bed.
The second and third regime rivers are in relatively flat deltaic landscapes, so other factors control where sedimentation leads to avulsion. In flat landscapes, a river’s current slows as it approaches the sea or a lake downstream, allowing sediment to build up.
Sediment deposition is interrupted by flooding, which causes erosion that propagates upstream like a reverse domino effect. For many years, deposits during low flows combine with erosion waves during floods to fill the channel at a particular location, triggering an avulsion.
The main difference between the rivers of the second and third regime is the distance traveled by erosion waves during floods.
Previous work by the group has suggested that erosion during floods is often limited to the backwater zone of a river, resulting in backwater-scale avulsions – the second regime avulsions. However, if the wave is moving fast enough and the floods last long enough, then a river can experience this increased sedimentation extremely far inland, leading to the third avulsion regime.
The group’s numerical models suggested third-regime rivers could exist, but it took them years to find examples. The team had focused on large rivers, such as the Mississippi and the Yellow River. It would take a flood that would last for centuries for a wave of erosion to travel the full length of the eddies of great rivers like these.
In contrast, it might only take a few days to a few weeks in some of Madagascar’s steep silt-bed rivers. The global dataset revealed that the extreme examples from Madagascar were far from mere outliers: two out of five avulsions on deltas fell into this category.
Humans and rivers under climate change
The dataset and results are more than academic considerations. “About 330 million people live in river deltas around the world, and many more live along river corridors,” Ganti said. “We need to understand how river mobility will change in response to climate change and anthropogenic interference.”
The team can now use their updated theory to predict avulsion locations, a matter of critical importance. Previously, scientists and officials might have assumed that locations upstream of the backwater area were safe. “But now we know there’s this other kind of avulsion on deltas where you’re not safe,” Chadwick said.
Additionally, changes in climate and land use may push avulsions inland on rivers under both delta regimes. The authors previously found that sea level rise and longer flooding can shift avulsion locations upstream from their historic locations. This means that even avulsions confined to the backwater zone could occur further upstream.
“That’s problematic because ‘a bit upriver’ on the Mississippi River is tens to hundreds of miles,” Ganti said. “It’s not something trivial.”
Agriculture, development, and resource extraction can also impact the location of avulsion. “If you change the land use – and therefore the amount of sediment supplied to certain rivers – you can take a river that is currently experiencing eddy-scale avulsions and move it into the category of inflow modulated avulsions. high sediment,” Ganti said.
This is the regime that can leap far upstream.
“This study clearly shows how sensitive the location of avulsion on deltas is to changes in sea level, sediment load, and flood duration and intensity…all of which are subject to change as the climate changes globally and more and more rivers are dammed, controlled and manipulated by human development,” said first author Sam Brooke, former postdoctoral researcher at UCSB.
This new framework allows the team to predict potentially dangerous inland migration from a river’s preferred avulsion site.
The team is currently studying the world satellite record to develop new measurements to characterize river mobility. They intend to use the observations to characterize factors that drive other types of fluvial mobility, in addition to avulsions. They also plan to study the frequency of avulsions.
“The question we’ve answered here is ‘where’ avulsions occur, but we haven’t really dig into ‘when,'” Ganti said. However, both are equally important to understand.
The group is excited about its future prospects and proud of the progress it has made so far. “It’s wonderful to see how far this has come,” Ganti said. “Between modeling, experiments and remote sensing, this is really a case where we picked a problem and approached it from all possible angles.”