Kosi River

The Kosi River is a major transboundary river system flowing through China (Tibet), Nepal and India. Draining the northern slopes of the Himalayas in Tibet and the southern Himalayan slopes in Nepal, it is among the most dynamic rivers in South Asia. Downstream of the major confluence north of the Chatra Gorge, the river is known as the Saptakoshi, a term that reflects its formation from seven significant tributaries. The river ultimately enters the state of Bihar in India, where it divides into distributaries before joining the Ganges near Kursela in Katihar district. Because of its chronic flooding and rapid shifts in course, the Kosi is often referred to as the Sorrow of Bihar.

Hydrography and Major Tributaries

The Kosi is the third-largest tributary of the Ganges by water discharge. Its drainage system covers extensive areas of Tibet, Nepal and the Indo-Gangetic plains of Bihar. Its seven upper tributaries include the Tamur, Arun and Sun Kosi, the latter fed by an east–west sequence of tributaries comprising the Dudh Kosi, Likhu Khola, Tamakoshi, Bhote Kosi and Indravati. These rivers traverse steep Himalayan terrain and converge at Triveni in Nepal, forming the Saptakoshi.
The combined river then flows through the narrow Chatra Gorge before debouching onto the plains. The catchment extends across six physiographic and climatic belts—from the Tibetan plateau and the high Himalayas to the Siwalik Hills and Terai. Notable peaks within the basin include Mount Everest, Kangchenjunga, Lhotse, Makalu, Cho Oyu and Shishapangma. The basin also contains numerous glaciers and glacier lakes, particularly within the Dudh Kosi subbasin, which houses 36 glaciers and 296 lakes.
In India, major tributaries such as the Kamala and Bagmati join the Kosi, along with smaller tributaries like the Bhutahi Balān.

Geomorphology and Channel Dynamics

The Kosi is widely recognised for its unstable channel behaviour. Historically, various authors proposed dramatic westward shifts of over one hundred kilometres during the past two centuries. However, a review of twenty-eight historical maps indicates that movement has been more subtle, with slight eastward tendencies and oscillatory lateral shifts. The underlying cause of instability lies in the vast Kosi alluvial fan, one of the largest in the world. This fan stretches over several thousand square kilometres below the Siwalik Hills and is dissected by more than a dozen shifting channels.
The river’s propensity for avulsion stems from steep gradients, intense monsoonal flows and the enormous sediment load derived from active erosion in the Himalayas. As the river emerges from the Chatra Gorge—a confined, high-energy section—it deposits vast quantities of silt, raising its bed and predisposing it to sudden course changes. Satellite imagery and historical records show abandoned channels extending across the fan, including former confluences with the Mahananda River prior to the eighteenth century.

Flooding and Human Impact

Flooding is a defining feature of the Kosi system, particularly in Bihar where annual inundation affects extensive tracts of fertile agricultural land. These recurrent floods disrupt rural economies and pose severe humanitarian challenges. The river’s average discharge is extremely high, reflecting both monsoon precipitation and glacier melt.
A major flood event occurred in August 2008 when the Kosi unexpectedly reverted to an old channel near the Nepal–India border after breaching its embankment at Kusaha in Nepal. This diversion routed around 95 per cent of the river’s flow through a previously abandoned course. Approximately 2.7 million people were affected across several districts of Nepal and India, including Supaul, Araria, Saharsa, Madhepura, Purnia and Katihar. Rescue operations required extensive mobilisation of the Indian Armed Forces and national disaster response agencies. The flood was declared a national calamity in India.
Smaller-scale floods continue to occur due to monsoon-driven avulsions, embankment failures and siltation. Declining fish populations and outmigration of local youth reflect broader socio-economic pressures associated with the river system.

The Kosi Project and Flood Control Measures

Following the catastrophic floods of 1954, India adopted a National Flood Control Policy that identified a multipronged strategy for managing the Kosi basin. Investigations between 1946 and 1955 led to the conception of the Kosi Project, which consisted of three interlinked components:

• Construction of the Koshi (Bhimnagar) Barrage, designed to stabilise the river near the Nepal–India border, provide irrigation and hydropower benefits and anchor the river’s shifting course.
• Embankments upstream and downstream of the barrage, aimed at constraining the river within defined channels.
• A proposed high multipurpose dam at Barakshetra in Nepal, intended to deliver substantial flood moderation, irrigation benefits and hydroelectric power.

The first two components were implemented under the terms of the 1954 Kosi Agreement between Nepal and India, revised in 1966. The high dam, however, faced political and environmental challenges and remained unrealised for decades, though renewed discussions have been initiated under modified proposals.

Koshi Barrage and Associated Infrastructure

Constructed between 1959 and 1963, the Koshi Barrage lies astride the Indo–Nepal border and was financed entirely by India. The barrage regulates flows into two major irrigation canals: the Eastern Canal, designed to irrigate large tracts in Nepal and India and support a 20 MW hydropower plant, and the Western Canal, which provides irrigation to additional areas in Nepal.
The barrage includes a roadway that has become an important transport link, forming part of Nepal’s east–west highway. Downstream, an inundation canal at Chatra provides irrigation to about 860 square kilometres in Nepal. Since Nepal assumed operational responsibility in 1976, the barrage and associated systems have undergone modernisation with international assistance.

Management Challenges and Regional Significance

The Kosi’s transboundary nature and hydrological volatility pose ongoing challenges for water management and disaster resilience. The river influences the livelihoods of millions in Nepal and India, while its upstream reaches interact with the hydrological systems of Tibet. Extreme sediment loads, rapid geomorphological change and the risks posed by glacial melt complicate engineering interventions. Embankments, although designed to mitigate flooding, can exacerbate vulnerability by raising the riverbed and amplifying the consequences of breaches.