In this guide
On 18 August 2008, the Kosi broke through its eastern embankment at Kusaha in Nepal, a few kilometres upstream of the barrage at Bhimnagar. The river abandoned its channel and poured into an old course to the east, flooding districts such as Supaul, Madhepura, Saharsa, Araria and Purnia. Lakhs of people were displaced; villages, roads and fields disappeared under water and sand. It was one of the worst disasters in Bihar's recent history — and a reminder of why the Kosi is called the Sorrow of Bihar.
The river
- The Kosi is formed in Nepal by seven Himalayan streams — hence Sapta Kosi — including the Sun Kosi, Arun and Tamur. Its catchment includes some of the world's highest peaks, including the Everest region.
- It enters Bihar near Bhimnagar (Supaul district) and flows south to join the Ganga near Kursela (Katihar district).
Why the Kosi is so difficult
| Factor | Explanation |
|---|---|
| Enormous sediment load | The Kosi carries one of the highest sediment loads of any river in the world, from the young, easily eroded Himalaya |
| Sudden drop in gradient | When it leaves the steep mountains and enters the flat plain, the river slows and drops its sediment |
| Aggradation | Sediment raises the riverbed, so the river spills over its banks |
| Shifting course | The river repeatedly abandons its raised bed for lower ground |
| Heavy monsoon rain | In the Nepal catchment, rainfall is intense and concentrated |
| Seismic zone | The region is earthquake-prone, which affects slopes and sediment supply |
The Kosi megafan
Over thousands of years, the shifting Kosi has built a huge, fan-shaped deposit of sediment — the Kosi megafan — one of the largest in the world. Over roughly the last two centuries, historical maps show the river shifting westward by well over a hundred kilometres across this fan.
The Kosi project
After independence, India and Nepal signed the Kosi Agreement in 1954. The Kosi project included:
- the Kosi barrage at Bhimnagar (on the Nepal border), completed in the early 1960s;
- embankments on both sides of the river to confine it to a fixed channel;
- canals for irrigation (the Eastern and Western Kosi canals);
- a small hydropower component.
The embankment debate
Embankments did stop the Kosi from shifting for decades. But experts have long pointed to problems:
| Problem | Explanation |
|---|---|
| Silt trapped between embankments | The riverbed rises between the embankments, sometimes above the surrounding land |
| Villages inside the embankments | Several hundred villages lie between the embankments, flooded almost every year, with poor services |
| Waterlogging outside | Embankments block natural drainage, causing waterlogging in the countryside outside |
| Breaches | When an embankment breaks, the damage is catastrophic, because water rushes into areas that have grown complacent — as in 2008 |
| Maintenance | Maintaining long embankments on shifting sands is difficult and costly |
Researchers and activists such as Dinesh Kumar Mishra have documented these problems in detail.
The 2008 Kusaha breach
- Cause: a breach in the eastern embankment at Kusaha, in Nepal — attributed to weaknesses in the embankment and inadequate maintenance and monitoring, at a time of relatively moderate flow.
- Effect: the river took an old course to the east, flooding large parts of north-east Bihar.
- Response: major rescue and relief operations; later, reconstruction under the Kosi Flood Recovery Project with World Bank support.
- Lessons: the importance of maintenance, monitoring, early warning, and cooperation with Nepal.
Options for the future
| Option | Pros | Cons |
|---|---|---|
| High dam in Nepal (the proposed Sapta Kosi High Dam) | Could store water, trap sediment, generate power and control floods | Very costly; seismic risk; displacement in Nepal; sediment would fill the reservoir; needs agreement with Nepal |
| Stronger embankments | Protects existing investments | Does not solve siltation |
| Living with floods | Flood-resilient housing, raised platforms, early warning, flood insurance, floodplain zoning | Requires changes in settlement and livelihoods |
| Drainage improvement | Reduces waterlogging | Needs sustained investment |
| Sediment management | Dredging; allowing controlled flooding to spread silt | Technically challenging |
| River linking (Kosi–Mechi) | Diverts water | Limited effect on floods; costly |
Most experts now favour a flood management approach — reducing losses and helping people live with floods — rather than only trying to control the river.
Why the Kosi matters for BPSC
- It is a classic Prelims and Mains topic.
- It links geography (sediment, megafans), disaster management (2008), international relations (India–Nepal agreements), economy (the Kosi region is among Bihar's poorest) and migration.
Practice
Prelims-style:
- Why is the Kosi called the Sapta Kosi?
- When was the Kosi Agreement signed with Nepal?
- Where is the Kosi barrage?
- Where did the 2008 breach occur?
- In which direction has the Kosi shifted over the last two centuries?
Mains-style:
- "Embankments have turned the Kosi from a problem into a disaster." Critically examine.
- Suggest a sustainable strategy for managing floods in the Kosi basin.
A note on dates and numbers. Exam patterns, vacancies and schedules change from year to year. Always confirm the current details in the latest notification on the Bihar Public Service Commission website .
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