Koshi (Sapta Koshi)

The Koshi is Nepal's largest river system — the Sapta Koshi, 'seven Koshis', gathering the Sun Koshi, Tama Koshi, Dudh Koshi, Likhu, Indrawati, Arun and Tamor. The three biggest trunks — the Sun Koshi from the west, the Arun out of Tibet, and the Tamor from the Kanchenjunga country in the east — meet at Tribeni, and a few kilometres downstream the combined river saws through the last hill ridge at the Chatara gorge and spreads onto the Tarai, crossing into India at Bhimnagar and running on across the Bihar plains to the Ganga.

No other river basin on Earth reaches so high. The Koshi's ≈74,500 km² catchment, spread across Tibet, Nepal and India, takes in both Mt Everest and Kanchenjunga, and its mean flow near the Ganga is on the order of 2,166 m³/s. The average hides ferocious monsoon swings: eroding some of the world's steepest slopes, the river carries one of the heaviest silt loads of any river its size, and has built an alluvial megafan of roughly 15,000 km² across the plains, over which it has repeatedly swung its channel — wandering behaviour that earned it the name 'Sorrow of Bihar'.

Taming the Koshi has been a joint Nepal–India enterprise since the Koshi Agreement of 25 April 1954 (revised in 1966), under which the 1,150 m, 56-gate Koshi Barrage was built at Bhimnagar in 1958–62, flanked by long embankments meant to pin the channel. On 18 August 2008 the river burst through the eastern embankment at Kusaha in Sunsari — inside Nepal, upstream of the barrage — and poured into a channel it had abandoned generations earlier. Roughly 95% of the river's flow left its engineered course; about 2.3 million people were affected in Bihar and over 50,000 in Nepal, with more than 400 dead, making it one of South Asia's worst river disasters of recent decades.

Just above the barrage lies Koshi Tappu, Nepal's first Ramsar wetland (designated December 1987, 176 km²), holding the country's last wild water buffalo — about 430 at the 2016 count — and 485 recorded bird species. Upstream, the basin anchors much of Nepal's hydropower, from the 456 MW Upper Tamakoshi to the 900 MW Arun-3, and carries the country's classic long rafting run down the Sun Koshi. ICIMOD has run basin-wide programmes here for over a decade, calling the Koshi the lifeblood of millions across China, India and Nepal.