Avulsion in rivers: Kosi River course change not natural; here’s why

What is the News?

According to a new study, rivers lining tropical and desert regions are more likely to change directions. These rivers expose the surrounding areas to floods as they abandon their usual route for a new one.

What is avulsion?

Avulsion means a sudden cutting off of land by flood or change in course of a body of water. Avulsions are rare, occurring only once a decade or century, or even less.

Avulsions are infrequent in nature compared to more frequent extreme weather events and the effect of sea-level rise.

What are the key findings of the study on avulsions?

The study documented satellite imagery from 1973-2020 and historical maps of 113 avulsions worldwide and categorized them into three types.

Type 1: According to the study, in 33 instances, rivers changed routes in the bases of mountains while descending onto unconfined valleys or open oceans. For example, the Kosi river belongs to this category.

Type 2:  Along backwater zones, part of the river flows differently because of the effects of the downstream sea. They documented 50 such instances occurring on low sloping deltas along some of the world’s largest waterways, like the Orinoco, Yellow, Nile and Mississippi Rivers.

Type 3: Occurs in rivers with extreme sediment load.

What are the reasons for avulsions?

The findings of the study predicted various factors for avulsions. Such as,

-The study found a common factor in all three categories as sediments. Sediments are known to fill up river beds, forcing rivers to seek new channels during floods.

Kosi-like systems bring a lot of sediments from the Himalayas. After embankments were made on either side of the river in the 1950s, it became much more unstable.

Before the embankment, the river could distribute sediments along the 200-kilometre stretch. Now that has been reduced to 10 km. Hence, the area available for its movement has gone down.

-Rising sea levels can push avulsions farther inland in the backwater zone.

-Temporary solutions like embankments exist. Such temporary solutions contribute to a false sense of protection and even amplify degradation at the system scale by limiting natural sediment dispersal.  This distributes the flow of the water and sediments across channels, dissipating floods and avulsion.

What is the significance of the findings?

Around 330 million people live on river deltas worldwide, and many more live along river corridors. It is essential to understand how river mobility will change in response to climate change and anthropogenic interference.

In India, the Kosi River abandon its established channel for an older one in 2008. This resulted in displacing 3 million people and claiming more than 250 human lives.

Source: The post is based on the article “Kosi River course change not natural; here’s why” published in DTE on 27th May 2022.