Why Is The Kosi So Prone To Flooding? The ‘Sorrow Of Bihar’ Explained

The Kosi River is among the most flood-prone rivers in the Himalayan region. Its steep mountain catchment, intense monsoon rainfall, heavy sediment load and unstable channel system combine to make flooding a recurring hazard. Known as the “Sorrow of Bihar”, the river has repeatedly changed course across the plains, affecting communities in both Nepal and India.

The Kosi basin stretches from the Tibetan Plateau through Nepal into the Gangetic plains of India. It covers about 87,481 sq km and is home to almost 40 million people whose lives and livelihoods depend on its water and land resources.

Why Does Kosi Flood So Easily?

The basin's mountainous terrain and intense monsoon rainfall make it highly vulnerable to erosion, landslides and sedimentation. ICIMOD says that although the Kosi basin represents only about 9% of the Ganges river system, it contributes nearly 25% of its total sediment load, estimated at 100-135 million tonnes a year.

Much of that sediment is produced upstream and transported into the plains of southern Nepal and northern India. The resulting riverbed aggradation contributes to frequent flooding and bank erosion and can increase the likelihood of the river shifting course.

A River That Changes Course

The Kosi's channel has shifted significantly across its alluvial fan. Research archived by ICIMOD has estimated that the river moved more than 113 km across the plains over roughly two centuries, although the nature of that movement has been debated in subsequent geomorphological research.

The 2008 Kosi flood demonstrated the scale of the risk. A breach near Kusaha in Nepal triggered a major avulsion, shifting the river about 120 km eastward. ICIMOD says the disaster displaced more than 70,000 people in Nepal and affected more than four million people in India.

The basin's geography adds to the danger: the river descends from some of the world's highest mountains into the relatively flat plains, creating major differences in slope and river behaviour along its course.

Why Is Sediment So Important?

Sediment is central to the Kosi's flood risk. ICIMOD estimates that about 20 million tonnes of sediment are deposited each year between Chatara and Birpur, with further deposition downstream.

This accumulation can alter the river's channel and contribute to flooding and bank erosion, particularly during the monsoon.

Why ‘Sorrow Of Bihar’?

The nickname reflects the river's long history of severe flooding in Bihar. The Kosi has been called the “Sorrow of Bihar” because of the extensive flood damage it has caused.

Because the Kosi is transboundary, processes in its Himalayan catchment can influence river conditions hundreds of kilometres downstream, where much of the flood damage occurs. Its combination of intense monsoon rainfall, steep terrain, enormous sediment flows and a highly mobile channel makes the river particularly difficult to control and predict.