Tama Koshi (Tamakoshi)

The Tama Koshi (also written Tamakoshi or Tamba Koshi) is a Himalayan river of east-central Nepal and one of the principal northern tributaries of the Sun Koshi within the wider Koshi (Saptakoshi) river system. It runs for about 92 kilometres (57 miles). Its headwaters lie across the border in the Tibet Autonomous Region of China, where the river forms from the Rongshar Chu (Rongshar Tsangpo) and the Lapchi Gang, which descend from the high glaciated terrain along the Nepal–Tibet frontier and enter Nepal near the village of Lamabagar in upper Dolakha District.

From Lamabagar the Tama Koshi flows broadly southward through the deep gorges of Bagmati Province, draining Dolakha District and then Ramechhap District before it joins the Sun Koshi. The basin is one of dramatic relief: catchment elevations range from roughly 850 metres in the lower valleys to nearly 7,000 metres on the surrounding ridges, and the watershed is flanked by celebrated peaks of the Rolwaling Himal, including Gauri Shankar (about 7,134 m) and Melungtse. This steep, high-altitude topography gives the river a swift, boulder-strewn character and makes its valley a corridor for trade and trekking routes toward the Rolwaling region and the Tibetan border.

Estimates of the drainage area vary with the gauging point. Studies place the basin above the settlement of Busti at about 2,753 square kilometres and the catchment above the Upper Tamakoshi dam site at Lamabagar at roughly 1,745 square kilometres, of which a large share lies inside Tibet. The whole Tama Koshi basin is commonly cited at around 2,900 square kilometres.

The Tama Koshi is a snow- and glacier-fed mountain river, so its flow is strongly seasonal: discharge swells during the summer monsoon (roughly June to September) and during the spring–summer snow and ice melt, then falls back through the drier winter months. Glaciers in the upper basin contribute a meaningful share of the annual runoff, sustaining dry-season flows that are important for the run-of-river hydropower plants downstream.

Along its course the river gathers several tributaries. On the left bank it receives the Rolwaling Chu, which drains the Rolwaling valley and the glacier-fed lake of Tsho Rolpa, and the Khimti Khola, which marks part of the boundary between Ramechhap and Dolakha districts; the Chyadu Khola enters from the right. The Tama Koshi ultimately joins the Sun Koshi, and within the larger Koshi system its waters travel on through the Sun Koshi to the Saptakoshi — the great river formed in eastern Nepal from seven Himalayan tributaries, among them the Sun Koshi, Indrawati, Tama (Tamba) Koshi, Bhote Koshi, Dudh Koshi, Arun and Tamur.

The Tama Koshi's combination of steep gradient and reliable flow has made it one of the most important hydropower rivers in Nepal. Its showpiece is the Upper Tamakoshi Hydroelectric Project, a 456 MW peaking run-of-river plant at the head of the valley in Dolakha District — the largest power station in the country. The scheme exploits a natural hydraulic head of about 822 metres, diverting water from a low dam (about 22 m high and 60 m wide) at Lamabagar through roughly 16 kilometres of headrace and tailrace tunnels to an underground powerhouse fitted with six Pelton turbines, with design generation of around 2,281 GWh a year.

Developed by Upper Tamakoshi Hydropower Limited, a company set up by the Nepal Electricity Authority, the project was widely promoted as a 'people's' or national hydropower venture, with public institutions and citizen-investment schemes among its shareholders. Construction began around February 2011 but was beset by delays — including the 2015 earthquake, contractor problems and large cost and interest overruns — and the plant was finally inaugurated on 5 July 2021 by Prime Minister K. P. Sharma Oli. When running at full capacity it can supply a substantial fraction of Nepal's electricity, helping to cut load-shedding and reduce power imports.

The river also hosts other plants. The Khimti I project (60 MW), commissioned in 2000 on the Khimti Khola tributary, was the first major private-sector (independent power producer) hydropower scheme in Nepal and generates on the order of 350 GWh a year. Smaller and cascade schemes such as Sipring Khola and the planned Tamakoshi V — which is designed to draw water from the Upper Tamakoshi tailrace rather than build its own dam — further extend the river's role in the national grid. Beyond power, the river's waters and valley support local irrigation and farming in Dolakha and Ramechhap.

Like other Himalayan rivers in Nepal, the Tama Koshi carries strong religious associations within the Hindu and local cultural landscape of its valley. Its upper catchment lies in the wider Dolakha region near Kalinchowk, whose Bhagwati temple is one of the most visited Shakti shrines of the area, and folk tradition links the surrounding ridges and sacred high-altitude ponds to the worship of the mother goddess.

Riverside ghats near confluences along the Tama Koshi and the lower Sun Koshi serve as sites for Hindu rituals and bathing, particularly during festivals such as Maghe Sankranti and the Dashain season. Riverine and fishing communities, notably the Majhi, observe their own water-centred rites — including worship of the river and rituals tied to riverside cremation and life-cycle ceremonies — reflecting the river's central place in everyday livelihood and belief along its banks.

The Tama Koshi basin is exposed to several of the natural hazards typical of the high Himalaya, and these directly threaten the people and the hydropower infrastructure along the river. The most studied danger is the glacial lake outburst flood (GLOF) risk from Tsho Rolpa, one of Nepal's largest glacial lakes, which sits high in the Rolwaling valley and drains via the Rolwaling Chu into the Tama Koshi. Concern over a sudden breach led, in the late 1990s, to engineering works that lowered the lake level by about three metres and to the installation of an early-warning system along the Rolwaling and Tama Koshi valleys; a major outburst could endanger downstream settlements, roads and power facilities across Dolakha, Ramechhap and neighbouring districts.

The valley is also seismically active and prone to landslides and debris flows, as demonstrated when the April 2015 Gorkha earthquake damaged roads, bridges and works at the Upper Tamakoshi site and forced the evacuation of workers. Steep, fragile slopes, intense monsoon rainfall and ongoing glacier retreat under a warming climate combine to make flooding, slope failure and sediment surges persistent concerns, and they feature prominently in the climate- and disaster-risk assessments prepared for the river's hydropower projects.