Trishuli

The Trishuli is one of the principal rivers of central Nepal and the headwater system closest to the Kathmandu Valley. It is formed by the merger of two trans-Himalayan streams that rise in Gyirong (Kyirong) County of Tibet — the Kyirong Tsangpo and the Lende Khola — whose combined flow enters Nepal near the historic border post of Rasuwagadhi. The catchment of these source streams lies in the Pekhu Kangri range, known on the Nepali side as the Langtang Himal, so the river drains some of the most heavily glaciated terrain in the central Himalaya.

From the border the river runs broadly southward through Rasuwa, Nuwakot and Dhading districts before reaching the lowlands. By convention the river takes the name Trishuli after it is joined by the Trisuli Khola, a stream that descends from the sacred lake of Gosaikunda above Dhunche. Below the town of Bidur it gathers the Tadi Khola and the Likhu Khola, and for much of its lower course it is closely followed by the Prithvi Highway, the main road link between Kathmandu and Pokhara.

The Trishuli is a major tributary of the Narayani (Gandaki) system. It finally joins the Narayani at Devghat, the revered confluence on the edge of the Chitwan plains, after which the combined river flows south across the Terai and on to the Ganges in India. Estimates of the river's length vary by source, but it is commonly reckoned at roughly 185–200 kilometres from its trans-Himalayan source to the confluence.

The Trishuli drains a large, high-altitude basin. About 85 per cent of its catchment area of around 4,640 square kilometres lies above 3,000 metres in elevation, and roughly 11 per cent lies above 6,000 metres, which means a substantial part of its flow is fed by snow and glacier melt as well as by the summer monsoon. This high-mountain character gives the river a strongly seasonal regime, with low, clear flows in winter and spring and powerful, sediment-laden discharge during the June-to-September monsoon. A long-running hydrological monitoring station sits at Betrawati, at an elevation of about 600 metres.

The river receives numerous tributaries along its course. On the upper river the Langtang Khola joins the Bhote Koshi branch in the gorges of Rasuwa, and downstream the Tadi Khola and Likhu Khola add to the flow near Bidur. In its lowermost reach the Trishuli is itself joined by several of the great rivers of the western and central Gandaki basin — including the Budhi Gandaki, the Marsyangdi and the Seti Gandaki — before it merges into the Narayani at Devghat, making it a key gathering channel of the wider Gandaki river system.

The Trishuli is one of Nepal's most important rivers for hydroelectric generation, valued for its steep gradient, reliable melt-fed flow and proximity to Kathmandu. The original Trishuli Hydropower Station, commissioned in 1967 at Trishuli Bazaar in Nuwakot, was developed jointly by the governments of India and Nepal and was for many years one of the country's principal generating plants; it is a run-of-river station that has since been upgraded, and the downstream Devighat station operates as a cascade project drawing on the same flow.

In the twenty-first century the upper river has become the focus of much larger projects. The Upper Trishuli-1 (UT-1) project, located in Rasuwa about 70 kilometres north-east of Kathmandu, is a 216-megawatt run-of-river scheme being built by the Nepal Water and Energy Development Company under a build-own-operate-transfer agreement signed with the government in 2016. Its diversion dam sits in a narrow gorge a short distance below the confluence of the Langtang Khola and the Bhote Koshi, and the plant is designed around three Francis turbine units. The project is among the largest private hydropower investments in Nepal's history. Beyond power, the river and its tributaries also supply water for irrigation of farmland in the valleys and the Terai downstream, and the Prithvi Highway that follows the river is itself a major artery of the national economy.

The river's very name carries religious meaning: Trishuli derives from the trishula, the three-pronged trident of the Hindu god Shiva. According to the legend attached to the river's headwaters, Shiva drove his trident into the ground at Gosaikunda, the high alpine lake above Dhunche, causing three springs to gush forth that fed the Trisuli stream from which the river takes its name. Gosaikunda itself is a major Hindu pilgrimage site, drawing large crowds of devotees during the Janai Purnima full-moon festival, and this gives the river a sacred association from its very beginnings in the high mountains.

The Trishuli's religious significance is greatest at its end. The confluence at Devghat, where the Trishuli meets the Kali Gandaki to form the Narayani, is one of the holiest river junctions in Nepal. It is a celebrated place of pilgrimage, ritual bathing, cremation and the performance of last rites, especially during Maghe Sankranti, when thousands of pilgrims gather to bathe at the sangam. As with other great rivers of the Himalaya, the Trishuli is regarded as sacred along much of its course, and riverside ghats and shrines mark places of worship between the mountains and the plains.

The Trishuli is by far Nepal's most popular whitewater rafting river, owing in large part to its accessibility — the rafting reaches lie directly along the Prithvi Highway, only a few hours' drive from Kathmandu and on the way to Pokhara and Chitwan. Day and multi-day trips typically put in around Charaudi and take out near Kurintar or Mugling, running a stretch of gorges, pools and wave trains rated broadly Class III–IV depending on the season and water level. Named rapids along the run, and reliable flows through much of the year, have made it an introductory and family-friendly run that also offers exciting whitewater when the river is high. Its convenience has made rafting a significant part of the local adventure-tourism economy.

The river also poses real hazards. As a steep, monsoon-fed Himalayan river it is prone to rapid rises, flash flooding and high sediment loads in the wet season, and the gorges that make it scenic also make it dangerous when in spate. The Prithvi Highway that hugs its banks is one of Nepal's most accident-prone roads, and vehicles that leave the highway have on occasion plunged into the river, sometimes with heavy loss of life. Like other Nepali rivers, the Trishuli faces growing pressure from hydropower development, road construction, sand and gravel extraction and pollution, raising concerns about downstream flows, sediment regimes and aquatic habitat.