A sudden flash flood on the Nepal-Tibet border has killed hundreds of people, and more than 1,500 people, including 300 Indians, remain missing across the affected region.
The floods have raised a fresh alert in Bihar and Uttar Pradesh in India as the water levels in the Gandak and Kosi rivers have started rising. The Central Water Commission has warned of waves up to 3 metres and a possible flash flood in the Gandak.
Eight Bihar districts have been put on high alert. Uttar Pradesh’s Maharajganj and Kushinagar are also on alert.
But the Gandak is not the only Nepal-fed river that worries Bihar. The Kosi–also known as ‘River of Sorrow’–is also on the rise, and it does not just flood; it also changes its course, causing large-scale destruction
So, why is the Kosi so unstable? How can a river suddenly change its course? Why does it carry so much silt? Can embankments really control it? Let’s understand these questions in easy language.
Why is the Kosi so unstable?
The Kosi, known as the Saptakoshi in Nepal, is a network of rivers flowing through Tibet and Nepal before entering Bihar.
Its major tributaries include the Sun Koshi, Arun Koshi and Tamur Koshi. They meet at Triveni in Nepal and form the main Kosi system. The river then moves towards the plains of Bihar before joining the Ganga.
The biggest problem is the huge difference in terrain.
The river system flows from the high Himalayas, at elevations of around 8,000 metres, towards the plains at around 60 metres. This sharp fall in elevation gives the river enormous energy in the mountains.
But once it reaches the plains, its flow slows down. That is when another problem begins: sediment.
Why does the Kosi carry so much silt?
The Himalayas are a young and highly unstable mountain system. Erosion and landslides constantly send rocks, sand, mud and silt into rivers. The Kosi’s tributaries carry much of this material downstream.
When the river reaches flatter land, it loses the energy needed to carry such a heavy sediment load. The material then starts settling on the riverbed and floodplain.
A 2019 study by the International Centre for Integrated Mountain Development (ICIMOD) described the Kosi as a river with extremely dynamic channels and frequent flooding. The study estimated that the river carries around 100 million tonnes of sediment every year at Chatara in Nepal.
About 20 million tonnes is deposited annually between Chatara and Birpur near the India-Nepal border. This continuous deposition changes the river’s landscape.
What is the Kosi megafan?
Over thousands of years, the sediment carried by the Kosi has spread across a vast area. This has created what scientists call the Kosi megafan (a huge, fan-shaped sediment plain the river has built over time). It covers roughly 10,000 sq km.
Kosi is considered one of the world’s most dynamic river systems, as its megafan is still growing because the river continues to shift its course. This makes the Kosi different from many other flood-prone rivers.
In easy language, the river does not always stay where it is.
What is a shift in a river’s course and why is it dangerous?
The Kosi is known for sudden shifts in its course, also known as ‘avulsion’. It happens when sediment keeps building up in the riverbed. As the riverbed rises, water may find a lower and easier route through nearby land. The river can then break out of its existing channel and carve a new path.
In simple terms, the river can change where it flows. That makes Kosi floods especially difficult to manage. The danger is not just that water may cross the riverbank. The river itself can move towards a completely different area.
How did the 2008 Kosi flood show this danger?
The most serious example came in August 2008. An embankment breach at Kusaha in Nepal triggered a major avulsion. The Kosi shifted eastwards and entered an old channel. More than 20 lakh people were affected by the resulting floods.
The disaster showed why measuring only the water level is not enough in the Kosi basin. Authorities also have to consider the possibility that the river may change its course.
Why does the monsoon make the situation worse? The Kosi basin faces several natural hazards in its upper reaches. These include: 1. Cloudbursts — sudden and extremely heavy rainfall over a small area.
2. Landslides — large amounts of rock and soil moving down mountain slopes.
3. Glacial lake outburst floods (GLOFs) — sudden releases of water from glacial lakes.
4. These events can quickly send huge volumes of water and debris into the river system.
5. Changes in land use can add to the problem.
6. Deforestation and other changes can increase soil erosion and landslide risks.
By the time the water reaches Bihar, the Kosi may therefore be carrying both heavy monsoon flows and huge amounts of sediment.
Can embankments control the Kosi?
India and Nepal have jointly managed the Kosi under the Indo-Nepal Koshi Agreement since 1954. One of its key objectives is flood control. Embankments were built along the river in the 1950s and 1960s.
The idea was simple: keep the river within a defined channel and protect populated areas from floods.
But the Kosi creates a difficult engineering problem. The river continues to bring large amounts of sediment downstream.
When a sediment-heavy river is confined between embankments, sediment can accumulate within the restricted channel. This can raise the riverbed and increase pressure on the system.
The National Flood Commission’s 1980 report had also warned about the challenges of confining rivers such as the Kosi within embankments.
What is happening downstream in Bihar?
The immediate concern is the Gandak River, another major river flowing from Nepal into Bihar. Following the floods in Nepal’s Rasuwa district, water has been released from the Gandak Barrage at Bagaha.
All 36 gates of the barrage have been opened. At one point, more than 1 lakh cusecs of water was being released. The Central Water Commission has warned that the Gandak could see waves of up to 3 metres. There is also a risk of flash floods.
Eight Bihar districts are on high alert: 1. West Champaran
2. East Champaran
3. Gopalganj
4. Saran
5. Muzaffarpur
6. Vaishali
7. Sitamarhi
8. Sheohar
What happens next?
The immediate focus is on monitoring water levels in the Gandak and other rivers flowing from Nepal.
Authorities also need to watch for sudden changes caused by heavy rain, landslides and debris flows upstream. The larger lesson comes from the Kosi. Bihar’s flood risk is not only about how much rain falls or how high a river rises.
In rivers such as the Kosi, sediment, terrain and sudden changes in the river’s course can be equally important.
That is why the Kosi remains one of Bihar’s most difficult rivers to control. And as the latest Nepal floods show, what happens in the Himalayas can quickly become a downstream concern for Bihar.
