Throughout history, major cities around the world have prospered along riverbanks. But rivers can also be destructive forces. They routinely flood, and on rare occasions, they may suddenly change course.
These “canal jumping” events, called avulsions, have caused some of the deadliest floods in human history.
Outbreaks on the Yellow River of China killed more than 6 million people in the late 19th and early 20th centuries. Similar events have been linked to the decline of Mesopotamian civilization along the Tigris and Euphrates rivers in what is now Iraq, Kuwait, Turkey and Syria.
In a recently published study, I worked with colleagues to map the global distribution of avulsions on river fans and deltas. We have used satellite imagery of more than 100 rivers from 1973 to the present, providing half a century of bird’s eye views of global river development.
We discovered 113 river avulsion events in temperate, tropical, and dry climates. Of those events, 33 occurred alluvial fans. These landforms form when rivers flow out of mountains or canyons onto an open plain or into the ocean and spread, depositing dirt and gravel in a triangular-shaped area.
The other 80 events took place river deltas – fertile, low-lying areas where slower-moving rivers branch into many canals that drain into lakes or the ocean, creating networks of wetlands.
We used this new data set to answer a simple question: What determines where avulsions occur?
Water seeks the lowest path
Avulsions occur due to sediment deposition. Over time, rivers deposit sediment at the avulsion site, suffocating the river with sediment. Water always flows down, so as its current course becomes increasingly blocked, it then jumps to a new location.
Much like earthquakes, river eruptions occur periodically in the same places. They disperse sediment and water across the river plains of the rivers, producing the characteristic triangular shape of these formations.
One recent example occurred in 2008, when the Kosi River in India changed direction at more than 60 miles (100 kilometers) in a matter of days, displacing more than 3 million people.
In the United States, the Mississippi has changed course many times for the past 7,000 years. Today, a plurality control structure in central Louisiana prevents it from jumping off its banks and joining the Atchafalaya River, but scientists have warned that a mega-flood could overcome these barrierscausing widespread economic damage across southern Louisiana.
A river may not change course more than once in many decades, or even centuries. Scientists’ understanding of where these events occur is poor and largely depends on a handful of them detailed observations of large deltasplus laboratory and computer models.
Three types of avulsions
Our global database has revealed three distinct types of avulsions.
First, the 33 avalanches on alluvial fans occurred when the rivers left canyons. Once the rivers no longer flowed through limited valleys, they could spill to one side or the other to the lowest ground.
The 80 avulsions that occurred on deltas were affected by forces in their outposts. The isolation of a river is the area where the speed of the flow is affected by the presence of the ocean or lake at the end of the river. In this zone, the river flow either slows down or accelerates in response to changing flood conditions. Scientists can estimate the isolation length of the size and slope of the river.
For example, the Mississippi River has an isolated length of nearly 300 miles (480 kilometers), which means that the speed of its flow is affected by the Gulf of Mexico all the way to a point north of Baton Rouge, Louisiana. Steeper rivers can have a ebb and flow scale as short as 0.6 miles (1 kilometer).
When a river flows normally, it slows down in its isolation and drops sediment into the stream. However, when floods occur, the larger volume of faster moving water erodes the channel.
This effect begins at the mouth of the river and moves upstream, in the opposite direction of the flow of water, removing some of the sedimentation that formed before the flood.
Ultimately, this interaction between sedimentation and erosion causes the river to choke up with sediment at a location that roughly coincides with the isolation length.
Our database showed that 50 of the 80 avulsion events that occurred on deltas occurred approximately at the surface length.
For example, the Catatumbo River in South America changed direction in 1982 about 6.5 miles (10.5 kilometers) inland from the point where it flows into Lake Maracaibo in Venezuela – close to its isolation length, which is 8.5 miles (13.7 kilometers).
Some rivers can change course long upstream
However, we also discovered a new class of avulsions on deltas that reflected neither valley opening nor isolation length. These rivers changed course long upstream from the point where they were hit by the lakes or oceans at their mouths.
These deltas were either on steep tropical islands such as Madagascar and Papua New Guinea or in desert environments such as Eritrea. In these places, rivers carry exceptionally large amounts of sediment during floods.
When the rivers flood, they erode their beds starting at their mouths and working backwards far upstream, similar to large rivers like the Mississippi.
However, the combination of long typical flood periods and exceptionally high sediment loads during floods allows erosion to progress long upstream.
As a result, these rivers can change course well above the isolation zone where avulsions occur in large coastal rivers.
More water, more sediment
Our description of these three types of avulsions provides the first framework for predicting where rivers will change direction on fans and deltas worldwide. These findings have crucial implications, especially for river deltas that are home to about 340 million people worldwide.
Most deltas are only a few feet above sea level, and some are very densely populated, such as the Mekong and Ganges-Brahmaputra deltas.
Our results show that avulsion sites on deltas can move from their historic sites to new areas.
Rapid sea level rise can move aviation sites inland on deltas, exposing new communities to catastrophic flood risks.
We also found that rivers in our second group – those where avulsions occur in the back zone – can shift into the third group, where avulsions occur significantly further upstream. We find that this can occur if the typical duration of a flood on a river or the sediment supply of the river changes.
Land changes, such as converting forests to rural areas, also increase sediment loads. In my opinion, it is necessary to understand how such changes can affect dynamic, volatile river systems – and the people who live around them – well into the future.