Important cities around the world have flourished along river banks. The rivers can be destructive. On rare occasions, they can shift pathways.
Some of the deadliest floods in human history have been caused by these channel-jumping events.
Over 6 million people were killed byvulsions on the Yellow River in the 19th and 20th century. Similar events have been linked to the decline of Mesopotamian civilization in what is now Iraq, Kuwait, Turkey, and Syria.
I worked with colleagues to map the global distribution of avulsions on river fans. We used satellite images of over 100 rivers from 1973 to the present to provide a half-century of bird's-eye views of global river evolution.
There were more than 100 river avulsion events in different climates. 33 of the events were on fans. When rivers flow out of mountains or canyons onto an open plain or into the ocean, they deposit dirt and gravel in a triangle-shaped area.
There were 80 events on river deltas, fertile, low-lying regions where slower- moving rivers branch into many channels that empty into lakes or the ocean.
We used this data set to answer a simple question.
There arevulsions because of deposition. The avulsion site is where the rivers deposit their debris. As the current course becomes increasingly blocked, the water eventually jumps to a new location.
Similar to earthquakes, river avulsions happen in the same places. These formations are characteristic of the flood plains and produce a triangular shape.
In 2008, the Kosi River in India shifted its course in a matter of days, causing over 3 million people to be displaced.
The Mississippi River has changed course many times over the past 7,000 years, but a multi-dam control structure in central Louisiana keeps it from jumping its banks and joining with the Atchafalaya River.
Over many decades or even centuries, a river may not change course more than once. Scientists don't have a good understanding of where these events occur and rely on a few observations on large deltas and laboratory and computer models.
There are three different types of avulsions.
The 33 avulsions on the fans happened when the rivers left the canyons. Once the rivers no longer flowed through valleys, they were able to spill over to one side or another.
The 80vulsions that happened on the deltas were influenced by the backwaters. The backwater of a river is where the speed of the current is affected by the presence of the ocean or lake. The river current can either slow down or speed up in this zone. The backwater length can be estimated from the size and slope of the river.
The speed of the Mississippi River's flow is affected by the Gulf of Mexico all the way to a point north of Baton Rouge, Louisiana. The backwater length scale is as short as 0.6 miles.
When a river is flowing normally, it slows down in its backwater stretch and drops its debris onto the riverbed. The riverbed erodes when floods occur.
The effect starts at the river's mouth and goes upstream in the opposite direction from the water's flow.
The interplay between erosion and sedimentation causes the river to choke up at a location that coincides with the backwater length.
50 of the 80vulsion events that occurred on the deltas happened at the backwater length, according to our database.
The Catatumbo River in South America changed course in 1982 to be closer to the Lake Maracaibo in Venezuela.
There is a new class of avulsions on deltas that do not reflect valley confinement or the backwater length. The lakes or oceans at their mouths affected the course of these rivers.
There were steep tropical islands like Madagascar and Papua New Guinea, as well as desert environments like Eritrea. During floods, the rivers carry large quantities of silt.
Similar to large rivers like the Mississippi, the rivers erode their beds when they flood.
The combination of long typical flood durations and high sediment loads allows the erosion to progress far upstream.
These rivers can change course above the backwater zone where avulsions happen in large coastal rivers.
The first framework for predicting where rivers will change course is provided by our description of the three types of avulsions. River deltas are home to some 340 million people around the world and these findings have important implications.
The majority of deltas are a few feet above sea level, and some are very densely populated.
The results show that avulsion sites can move from historic locations to new ones.
Rapid sea level rise can move avulsion sites inland, exposing new communities to catastrophic flood risks.
The second group of rivers can shift into the third group if avulsions occur in the backwater zone. This can happen if the typical duration of flooding on a river changes.
Climate change is causing flooding in many parts of the world.
Land use changes, such as converting forests to farmlands, are increasing the amount of silt in the water. It is important to understand how changes to the river systems can affect the people who live around them.
Vamsi Ganti is an assistant professor at the University of California Santa Barbara.
This article is free to use under a Creative Commons license. The original article is worth a read.
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