Koshi (Sapta Koshi)

The Koshi, known in Nepal as the Sapta Koshi ("Seven Koshis"), is the largest river system of Nepal and one of the great trans-Himalayan tributaries of the Ganges. Its name reflects its defining feature: the system gathers seven major Himalayan rivers — the Sun Koshi, Tama Koshi, Dudh Koshi, Indrawati, Likhu, Arun and Tamur (Tamor); some listings substitute the Bhote Koshi for the Indrawati — which drain the eastern third of the Nepal Himalaya, including the catchments of Mount Everest and Kanchenjunga. The basin is among the highest-altitude major river basins on Earth, falling from the crest of the Himalaya, and the Arun arm rises far to the north on the Tibetan Plateau, so the system spans three countries: China (Tibet), Nepal and India.

The three principal arms — the Sun Koshi flowing in from the west, the Arun descending from Tibet through the centre, and the Tamur draining the Kanchenjunga country in the east — converge at Tribeni (Triveni), in eastern Nepal. From this confluence downstream the united river is properly called the Sapta Koshi. It then passes through the Chatra gorge, where it breaks free of the Mahabharat (Siwalik) hills and spills onto the flat Terai plain near the Koshi Barrage at Bhimnagar on the India–Nepal border.

Below the barrage the river enters the Indian state of Bihar, where, freed of its mountain confinement, it spreads across a vast, low-gradient alluvial fan before joining the Ganges near Kursela in Katihar district. The river is roughly 720–729 kilometres long from its Himalayan headwaters to its outfall in the Ganges. Because it descends from some of the highest mountains on the planet to one of the flattest, most densely settled plains, the Koshi carries an exceptional load of sediment, a fact that shapes almost everything about its behaviour downstream.

Hydrologically the Koshi is dominated by its seven snow- and monsoon-fed tributaries. The Arun, which rises beyond the main Himalayan crest in Tibet, drains by far the largest area — more than half of the entire Sapta Koshi catchment — though, because much of its basin lies in the dry rain shadow of the plateau, it contributes a smaller share of the water than its area would suggest. The Sun Koshi, fed by the Bhote Koshi, Indrawati, Tama Koshi, Likhu and Dudh Koshi, forms the central trunk into which the western rivers drain, while the Tamur brings down the runoff of the Kanchenjunga massif from the far east.

The combined system drains a basin of roughly 74,500 square kilometres across Tibet, Nepal and India and carries a long-term mean discharge generally cited in the range of about 2,100–2,500 cubic metres per second near its outfall to the Ganges. This makes the Koshi the third-largest tributary of the Ganges by discharge, after the Ghaghara and the Yamuna. As with all Himalayan rivers, flow is highly seasonal: the summer monsoon between June and September delivers the great majority of the annual water and sediment, and monsoon flood peaks can run many times higher than the mean, while winter and spring flows are sustained largely by snow and glacier melt.

The river's most consequential hydrological trait is its sediment load. Intense monsoon rainfall, steep slopes, active tectonics and frequent landslides in the upper catchment combine to produce one of the highest silt yields of any river in the world. Where the river leaves the hills and its gradient collapses on the plain, this sediment is deposited in enormous quantities, continually raising the riverbed and building the great Kosi alluvial megafan — the physical process that underlies the river's notorious tendency to flood and to change course.

The Koshi basin is central to the economies of both eastern Nepal and the north Bihar plain. The keystone structure is the Koshi Barrage at Bhimnagar, on Nepali territory close to the Indian border, built between roughly 1959 and 1963 under the Kosi Agreement signed by Nepal and India in 1954 (later revised in 1966 to address Nepali concerns). The barrage feeds large canal networks — the Eastern and Western Kosi canals — that irrigate extensive farmland in Bihar and, on the Nepali side, the Chatara/Sunsari–Morang command area, while long marginal embankments were raised to try to confine the river's wandering channel.

The tributaries of the Koshi system are also Nepal's richest hydropower frontier. The Tama Koshi corridor hosts the 456 MW Upper Tamakoshi, Nepal's single largest power plant, and the pioneering 60 MW Khimti scheme; the Arun corridor is being developed as a cascade led by the 900 MW Arun-3, set to become one of the country's largest plants; and the Sun Koshi trunk has long been a world-class rafting river as well as the donor for the Sun Koshi–Marin diversion, a National Pride Project that tunnels water into the Bagmati basin for Terai irrigation. The far larger run-of-river and storage potential of the eastern rivers remains only partly tapped.

The system's biggest single proposal is the long-discussed Saptakoshi High Dam, a multipurpose project near the Chatara gorge intended to combine flood control, large-scale irrigation in both Nepal and Bihar, and major hydropower generation. Successive joint Nepal–India studies have envisaged a very high dam producing on the order of 3,000 MW, though in more recent talks the two governments have discussed scaling down the proposed height and output. The scheme has been repeatedly delayed by technical, financial, seismic-risk and political concerns, including Nepali fears over the area of land that a high reservoir would inundate, and it has not been built.

The Koshi and its tributaries carry deep cultural and religious meaning across the eastern Himalaya and the Gangetic plain. The very name Sapta Koshi — the seven rivers united into one — echoes the Hindu reverence for sacred confluences (sangam), and the meeting point of the system's great arms at Tribeni (Triveni, literally "three streams") shares its name with other holy confluences across South Asia, marking the place where the waters become one river.

For the diverse communities of the basin — including Rai, Limbu, Sherpa, Tamang, Newar and Madhesi peoples in the hills and plains — the Koshi and its valleys have for centuries been corridors of settlement, trade and pilgrimage linking the high Himalaya to the Ganges. The river's tributaries thread together famous sacred and mountaineering landscapes, from the Khumbu of Everest drained by the Dudh Koshi to the Kanchenjunga country drained by the Tamur, giving the system a cultural reach far beyond its banks.

Downstream in Bihar the Kosi occupies a powerful place in regional consciousness, celebrated in folk song and literature as a living, capricious river often personified as a goddess that gives fertile silt with one hand and takes away homes and fields with the other. This dual character — life-giver and destroyer — runs through the cultural memory of the plains communities who live with its annual floods, and it is captured in the river's enduring popular epithet.

The Koshi is famous, above all, as the "Sorrow of Bihar" — a reputation earned by centuries of devastating and unpredictable flooding on the Bihar plain. Because the river deposits its immense sediment load where it leaves the hills, its bed steadily aggrades and the channel becomes unstable, repeatedly breaking its banks during the monsoon and carving new paths across its enormous alluvial fan. Historical accounts describe the river migrating tens of kilometres across the plain over the past two centuries, abandoning old channels and seizing new ones, with its floods affecting large areas of fertile farmland across north Bihar.

The most catastrophic recent event was the 2008 disaster. On 18 August 2008 the eastern Koshi embankment failed at Kusaha in Sunsari district, Nepal, a short distance upstream of the barrage, and the river broke out of its confined channel to reoccupy an old course it had abandoned more than a hundred years earlier. The breach sent the Koshi surging eastward across the plain, inundating large parts of Nepal's eastern Terai and northern Bihar; the flood is reported to have affected on the order of 2.3 to 2.7 million people and ranks among the worst flood disasters in the region's modern history. The episode underscored how heavily the densely populated plain depends on the upkeep of the river's embankments.

The lower Sapta Koshi also sustains one of Nepal's most important wetland ecosystems. The Koshi Tappu Wildlife Reserve, established in 1976 and covering about 175 square kilometres of floodplain in the eastern Terai, was created to protect the last surviving Nepali population of the wild water buffalo (Bubalus arnee), known locally as arna. In December 1987 Koshi Tappu became Nepal's first Ramsar-listed Wetland of International Importance. Its braided channels and marshes shelter the endangered Ganges river dolphin, gharial, scores of fish species and vast numbers of resident and migratory water birds, making the reserve a critical refuge within a heavily modified river system. Looking ahead, the basin faces added pressure from glacial retreat, glacial-lake outburst flood (GLOF) hazards in the high tributaries, and changing monsoon patterns under a warming climate.