Gandaki (Narayani / Sapta Gandaki)

The Gandaki — known in Nepal across its length as the Kali Gandaki in the high mountains, the Narayani in the Terai, and the Gandak after it crosses into India — is one of the three great river systems of Nepal, alongside the Koshi to the east and the Karnali to the west. It rises near the border with Tibet on the main Himalayan crest in Nepal's Mustang District, at the Nhubine Himal Glacier in the high glacial country near Damodar Kunda, at an elevation of roughly 6,268 metres (about 20,564 feet). From its source the river runs southward across the arid trans-Himalayan plateau before plunging through the heart of the Himalaya.

Its most celebrated stretch is the Kali Gandaki Gorge, which cuts between two of the world's fourteen eight-thousand-metre peaks: Dhaulagiri (8,167 m / 26,795 ft) to the west and Annapurna I (8,091 m / 26,545 ft) to the east. Measured from these summits down to the river, the gorge is among the deepest on the planet, and it is often described as one of the world's deepest river canyons. The high, dry valleys of upper Mustang above the gorge lie in the rain shadow of the Himalaya, giving the upper Kali Gandaki a desert-like landscape utterly unlike the lush hills downstream.

Below the mountains the river descends through the Middle Hills, where it is progressively joined by its major tributaries. After its confluence with the Trishuli River it emerges into the plains near Devghat, on the edge of Chitwan, and from this point downstream it is called the Narayani. The Narayani forms part of the western boundary of Chitwan National Park, a UNESCO World Heritage Site, before reaching the Nepal–India border at Triveni. Crossing into India as the Gandak, it flows roughly south-east across Bihar and finally joins the Ganges (Ganga) near Sonpur and Hajipur, opposite Patna. The full system runs for about 814 kilometres (506 miles) and drains a catchment of roughly 46,300 square kilometres (17,900 sq mi), the greater part of it in Nepal.

The river is traditionally called the Sapta Gandaki, the 'Seven Gandakis', after the seven principal Himalayan-fed rivers that combine to form it. These are usually given as the Kali Gandaki, the Trishuli, and the five main tributaries of the Trishuli — the Marsyangdi, Budhi Gandaki, Seti, Madi and Daraudi. Several of these — notably the Trishuli, fed by snows along the Nepal–Tibet frontier, and the Marsyangdi and Budhi Gandaki draining the Manaslu and Annapurna massifs — are themselves large rivers, so the combined Gandaki carries one of the heaviest flows of any river leaving the Nepalese Himalaya.

The system is fed by a mixture of monsoon rainfall, snowmelt and glacial melt, which gives it a strongly seasonal regime: discharge is modest through the dry winter and swells dramatically during the June-to-September summer monsoon. The river's average discharge is on the order of about 2,000 cubic metres per second, falling away in the lean season and rising enormously during monsoon floods. This contrast between lean season and flood season shapes everything from navigation and irrigation to the river's well-known sediment load, much of it eroded from the steep, fast-rising Himalayan headwaters.

The Gandaki basin holds a large, only partly developed hydropower resource. The flagship project on the river is the Kaligandaki 'A' Hydroelectric Power Station near Mirmi in Syangja District, a run-of-river plant commissioned in 2002 with an installed capacity of 144 MW from three 48 MW Francis turbines. For nearly two decades it was the single largest power station of any kind in Nepal, until it was surpassed by the 456 MW Upper Tamakoshi project; it remains a central pillar of the national grid. Numerous other plants on Gandaki tributaries such as the Marsyangdi add further generation.

Downstream, the river is the basis of one of South Asia's major irrigation works. Under a 1959 Nepal–India agreement, the Gandak Barrage was built across the river in the late 1960s at Valmikinagar (Triveni), close to the border. Its canal network irrigates large areas of farmland in Nepal's Terai as well as the Indian states of Uttar Pradesh and Bihar, and a hydropower station is attached to the canal system. The fertile alluvial plains watered by the Narayani and Gandak, together with the river's fisheries and its growing role in white-water rafting and tourism, make the system economically vital on both sides of the border.

The Gandaki is among the most sacred rivers of the Hindu world and is closely tied to the worship of Vishnu. The upper river is the main natural source of shaligrams (also spelled saligram) — black, often spiral-marked stones that are fossilised ammonites, the shells of marine cephalopods that lived when the Himalayan rock was still part of the ancient Tethys Sea. Because the rising Himalaya lifted these marine fossils to great altitude, they tumble down the Kali Gandaki and are gathered from its bed; Hindus revere them as aniconic (non-figurative) embodiments of Vishnu/Narayana, and they have been venerated across Hindu, Buddhist and Bon traditions for some two thousand years.

The river's headwaters lie close to two of the Himalaya's great pilgrimage sites: Muktinath, a temple sacred to both Hindus and Buddhists set high above the gorge, and the glacial lake of Damodar Kunda, itself a destination for devout pilgrims. The name 'Kali Gandaki' associates the river with the goddess Kali, and the name Narayani derives from Narayana, an epithet of Vishnu. At Devghat, where the Kali Gandaki and Trishuli meet to form the Narayani, the confluence is a revered tirtha (pilgrimage ford) where the faithful perform ritual bathing, ancestral rites and cremations, and large numbers gather for festivals such as Maghe Sankranti.

The Gandaki drains some of the steepest and most tectonically active terrain on Earth, and the same forces that built its spectacular gorge also make the basin prone to natural hazards. The fragile, fast-eroding Himalayan slopes shed huge quantities of sediment and are subject to landslides, while the monsoon can turn tributaries into violent flash floods. The catchment also contains glacial lakes whose moraine dams can fail, producing glacial lake outburst floods (GLOFs) that surge down the valleys; downstream, the lower Gandak is a flood-prone river that periodically inundates farmland in the Terai and Bihar.

Ecologically the system is significant: the Narayani's stretch along Chitwan National Park supports the endangered gharial crocodile and the Ganges river dolphin, and the river is an important fishery. These values face pressures from sediment trapping and flow alteration at dams and barrages, riverbed extraction of sand and stone, pollution, and the broader effects of climate change on Himalayan snow and ice. Retreating glaciers and shifting riverbeds are also reported to be making the prized shaligram fossils harder to find, linking the river's environmental health directly to its living religious tradition.