Where rivers jump course: First global compilation of river avulsions

Humans have always had a complex relationship with rivers. Osiris, the ancient Egyptian god of death and rebirth, was linked with the yearly flooding of the Nile.

In rare and catastrophic events known as river avulsions, large floods will sometimes force a river to jump course and forge a new path across the landscape. These events can wipe out entire cities along the largest waterways, yet they also create fertile deltas that have nurtured many societies.

The first global collection of river avulsions was published in the journal Science. A decade of theoretical and experimental work by the group corroborates the findings of the study.

The first unambiguous test of the theory is provided by this dataset, which shows there are three distinct regimes of avulsion on fans and deltas.

Researchers were mostly kept in the dark because of rarity and elusiveness. Scientists only observed a few before this paper. From those few case studies, they began to build a theory of where avulsions occur.

Only once a decade, once a century, or even less, can a river jump course. Scientists have to monitor a river for a long time to get useful data. Satellite imagery allowed the team to swap large spans of time for large swaths of space.

Although avulsions are very rare, you are going to get lucky on a few of them. The team got lucky more than 100 times while combing the satellite data and historical maps.

Fan avulsions.

Three regimes were formed due tovulsion behavior. 33 instances where rivers jumped course when they transitioned from steep, confined channels to flatter terrain were found by the team. The fans are usually located at the bases of mountains, where a river exits the canyon onto unconfined valleys or open oceans. Avulsions of this type require a 3-fold break in the river valley slope.

Backwater-scaled delta avulsions.

The backwater zone is the part of the river that flows differently because of the effects of sea level at the downstream end. It can be as far inland as 300 miles for the Mississippi River.

50 of the dataset's avulsions were covered by this second group. The Orinoco, Yellow, Nile and Mississippi Rivers are some of the largest waterways in the world. Most of the previous case studies fit this category.

The load on the delta is extreme.

The last regime encompassed the remaining 30 avulsions. There were avulsions far upstream in these rivers. It's really far.

The third regime had an avulsion length that was 14 times greater than the river. In some of the most extreme examples, this could stretch to over 50 times the backwater length.

The team first documented this behavior in a 2020 paper on Malagasy rivers.

Ganti said it was a third regime of avulsions. It accounts for 40% of the avulsions in the global dataset.

The riverbeds are dynamic.

Avulsions are related to transport. They happen when a river fills up. This chokes up the channel to the point where it jumps somewhere else. This happens when the slope changes and the flow slows down.

The second and third regimes are found in relatively flat deltaic landscapes, so other factors control where an avulsion occurs. In flat landscapes, a river's current slows down as it approaches the sea or a lake. Floods cause erosion that travels upstream like a domino effect. Avulsion is caused by deposition during low flows combined with waves of erosion during floods to fill the channel at a particular location. Waves of erosion travel far in the second and third regimes.

During floods, backwater-scaled avulsions can be caused by erosion that is limited to a river's backwater zone. If the wave travels quickly enough, and the floods last long enough, the river can experience enhanced sedimentation, which can lead to the third avulsion regime.

It took the group years to find any examples of rivers in the third regime. The team focused on the Mississippi and Yellow Rivers. It would take a long time for an erosional wave to travel the entire length of the major rivers. It might take a few days to a few weeks in some of the steep, silt-bedded rivers. Two in five avulsions on deltas fell into this category, as revealed by the global dataset.

Climate change affects humans and rivers.

Around 330 million people live on river deltas worldwide, and many more live along river corridors.

The updated theory can now be used to predict avulsion locations. Scientists and officials used to think that locations upstream of the backwater zone were safe.

Climate and land-use change can push avulsions inland on rivers. The authors found that rising sea level and longer floods can shift avulsion locations upstream of their historic locations. The Mississippi River is hundreds of kilometers upstream, so even avulsions confined to the backwater zone could occur farther upstream.

If you change land use you can shift a river that is currently experiencing backwater-scaled avulsions. The regime can jump far upstream.

The study makes clear how sensitive avulsion location is to changes in sea level, silt load, and flood duration and intensity, all of which are subject to change as climate changes globally and more rivers are dammed, controlled and manipulated by human development. The team can predict a potentially hazardous inland migration of a river.

The team is looking at the global satellite record to develop new metrics for river mobility. They intend to use the observations to understand the factors that drive other types of river mobility. They plan to investigate avulsion frequencies. Both are important to understand.

The group is excited by their future prospects and proud of the progress they have made so far.

More information: Sam Brooke et al, Where rivers jump course, Science (2022). DOI: 10.1126/science.abm1215 Journal information: Science Citation: Where rivers jump course: First global compilation of river avulsions (2022, May 26) retrieved 27 May 2022 from https://phys.org/news/2022-05-rivers-global-river-avulsions.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.