Context: 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.

Kosi River

• The Kosi is a trans-boundary river which flows through China, Nepal and India.
• It is a major tributary of the Ganges.
• It drains the northern slopes of the Himalayas in Tibet and the southern slopes in Nepal.
• From a major confluence of tributaries north of the Chatra Gorge onwards, the Kosi River is also known as Saptakoshi for its seven upper tributaries.

These include:

• Tamor River originating from the Kanchenjunga area in the east; Arun River; Sun Koshi; Dudh Koshi; Bhote Koshi; Tamakoshi River; Indravati.
• The Saptakoshi crosses into northern Bihar, India where it branches into distributaries before joining the Ganges near Kursela in Katihar district.

Historical Significance

• The Kosi was also called Kausika in Rigveda.
• Formerly known as 'Kausika,' named after the sage Vishvamitra, a descendant of the sage Kusika, is said to have attained the status of 'Rishi' on the banks of the river.

Protected areas

In Nepal two protected areas are located in the Koshi River basin.

• Sagarmatha National Park
• Koshi Tappu Wildlife Reserve

Causes of floods in Kosi River

• Kosi River has deposited 1082 million tonnes of silt in the last 54 years.
• The silt causes the level of riverbed to rise.
• As a result, the natural longitudinal (straight) course of the river is disturbed.
• Therefore, the river searches for a lateral path (left or right).
• As a result, it changes its course and breaches the embankments on the new path it has created.
• The breach of embankments causes floods.
• The mouth of the channels also gets choked too often causing shift in courses, bringing new areas under the grip of flood.
• This phenomenon is further attributed to the rise in the water level, which ultimately results into overtopping and breaches of banks and floodings in the basin area.

About 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.

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.

Key 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.

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.