The Kosi is a significant body of water that traverses through Tibet, Nepal, and India. Starting from the highest upland in the world situated in Tibet, the river gathers a large part of Nepal’s drainage before unveiling itself onto the Gangetic plains. The Sun Kosi, Arun, and Tamur are the three major tributaries that meet at one point just above a 10 km gorge that slices through the lower slopes of the Himalayas. The river then crosses into northern Bihar, India, where it splits into distributaries before merging with the Ganges near Kursela in Katihar district. Tagged as the “Sorrow of Bihar,” the Kosi is notorious for its annual floods that devastate approximately 21,000 sq. km. of fertile agricultural lands, disrupting the rural economy.
Understanding Avulsions
Avulsion is a process that results in a deviation in the flow of a river, leading to the abandonment of the earlier river channel and creation of a new one. Tropical and desert rivers are most susceptible to avulsions. They occur less frequently, perhaps once every decade or century, making them less talked about yet they can have catastrophic effects.
The Research Study on Avulsions
A recent study documents 113 avulsions worldwide based on satellite images from 1973 to 2020 and historical maps. Rivers tend to change routes at mountain bases as they descend onto unconfined valleys or open oceans, while avulsions can also take place in delta regions. The study highlights instances of avulsion in rivers with excessive sediment loads, where riverbeds fill up with sediments, compelling the rivers to explore new channels during floods.
The Case of the Kosi River
The Kosi River, laden with sediment from the Himalayas, has witnessed increased instability since embankments were constructed on its sides in the 1950s. A major flood in 2008 thrust the Kosi River to abandon its prior channel for an older one, displacing 3 million individuals and claiming over 250 lives. The 200 km stretch where the river used to distribute sediments was reduced to 10 km after the construction of embankments. While these structures may provide a false sense of safety, they intensify system-scale degradation by restricting natural sediment dispersal.
Moving Forward
Approximately 330 million people live on river deltas globally, and even more reside along river corridors. It is therefore imperative to comprehend how climate change and human activities impact river mobility. By understanding how climate change affects avulsions, we can forecast shifts in river courses in delta regions. Furthermore, recognizing the role of embankments or barriers created for flood protection can help us understand their influence on avulsion triggers.
Various engineering interventions could be employed to create additional channels for rivers to prevent floods and avulsions. This would manage the flow of water and sediments across channels, mitigating the hazardous effects of avulsions.
UPSC Civil Services Examination, Previous Year Question
A previous question asked in the UPSC Civil Services Examination required the identification of the tributaries of the Brahmaputra River. Dibang, Kameng and Lohit are all principal tributaries of the Brahmaputra, making all of them correct answers.