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The Kosi River is a major transboundary river of the Ganga River system, flowing through China (Tibet), Nepal, and India. It is widely known as the “Sorrow of Bihar” because of its frequent and devastating floods that affect large parts of northern Bihar.

Recent observations indicate that the river has shifted its course westward by more than 100 km over the past two centuries, highlighting its highly dynamic and unstable nature.

Geographical Features and Course

• The Kosi River originates in the Himalayan region of Nepal and Tibet and flows through Nepal before entering the plains of India.
• Formation: The river is formed by the confluence of three major Himalayan streams: Sun Kosi, Arun Kosi, Tamur Kosi. These rivers originate in the Himalayan mountains and join in eastern Nepal to form the main Kosi River.

Course of the River

• The river flows through mountainous terrain in Nepal and receives several tributaries.
• About 48 km north of the India–Nepal border, it passes through the Chatra Gorge in the Siwalik Hills.
• After emerging from the hills, it enters the alluvial plains of Bihar in northern India.
• Finally, it joins the Ganges River south of Purnea after travelling approximately 724 km.

Drainage Basin

• The Kosi basin covers an area of about 74,500 sq km, of which only around 11,070 sq km lies within India. Most of the catchment area lies in Nepal and the Himalayan region, which strongly influences the river’s hydrology.
• The basin’s sandy and fertile soils support agriculture, particularly the cultivation of maize (corn).

Major Tributaries

• The Kosi river system consists of seven major tributaries, which are collectively known as the Sapta Kosi (Seven Kosis): Sun Kosi, Tama Kosi (Tamba Kosi), Dudh Kosi, Indravati, Likhu, Arun, Tamur (Tamor)
• These tributaries drain large parts of the Himalayan watershed, contributing significant water and sediment to the river.

Why is the Kosi Called the “Sorrow of Bihar”?

Frequent Flooding: The Kosi River is notorious for severe floods in northern Bihar, causing large-scale damage to: agriculture, infrastructure, and settlements.

Heavy Sediment Load: The river carries large amounts of silt and sediment from the young and fragile Himalayan mountains. During the monsoon season, this sediment deposition raises the riverbed, increasing the likelihood of flooding.

Shifting Course: One of the most striking features of the river is its frequent change in course.

• Over the last 200–250 years, the river has shifted more than 100 km from east to west across the plains of Bihar.
• This phenomenon creates new floodplains while abandoning older channels.

Challenges Associated with the Kosi River

The dynamic nature of the Kosi poses several challenges:

• Recurring floods and displacement of populations
• Damage to agriculture and infrastructure
• Riverbank erosion and channel migration
• Transboundary water management issues between India and Nepal

Efforts such as embankment construction and flood-control projects have been implemented, but managing the river remains difficult due to its high sediment load and unstable course.