Dudh Koshi

The Dudh Koshi (Nepali: दूधकोशी, literally 'milk river') is a glacier-fed Himalayan river in eastern Nepal that drains the southern flanks of the Mount Everest massif. Its name comes from the milky, opaque colour the water takes on from the fine glacial silt — known as rock flour — that its tributaries carry down from the high Khumbu glaciers. Rising in the Solukhumbu district within Sagarmatha National Park, it is one of the principal headwater rivers of the great Koshi (Sapta Koshi) system that drains much of eastern Nepal before crossing into the Indian state of Bihar.

The river is generally taken to begin at roughly 5,100 metres above sea level among the meltwater lakes and glaciers just east of the Gokyo Lakes, gathering the outflow of the Ngozumpa, Gokyo and neighbouring glaciers. From these icy headwaters it flows south, descending steeply past the Sherpa heartland of the Khumbu. It runs by Namche Bazaar, the chief town of the region, exits the boundary of Sagarmatha National Park west of Lukla, the airstrip town that is the trekking gateway to Everest, and continues its southerly course through the middle hills.

The Dudh Koshi finally joins the Sun Koshi near Harkapur at about 1,245 metres above sea level — a total fall of roughly 3,800 metres from source to confluence. From that junction the combined waters form part of the Koshi (Sapta Koshi), the 'seven Koshis', whose major Himalayan tributaries unite in eastern Nepal. The river is frequently described as among the highest-rising rivers in the world by source elevation, and its upper gorge is steep, with rapids of the highest whitewater grade that have long attracted expedition kayakers.

The Dudh Koshi is fed almost entirely by snow and glacial meltwater from the Everest, Lhotse, Nuptse and Cho Oyu region, supplemented heavily by the summer monsoon. Its flow is strongly seasonal: discharge peaks in the monsoon months of June to September, when rain and accelerated glacial melt combine, and falls markedly in the dry winter. The characteristic milky turbidity that gives the river its name is greatest in the melt season, when suspended glacial flour is abundant.

Several glacier-fed tributaries build the river as it descends the Khumbu. The Imja Khola, draining the Imja and Lhotse glaciers and the valley below Dingboche and Pheriche (where it is joined by the Lobuche River), is a major tributary that merges with the upper Dudh Koshi near Phakding. The Bhote Koshi branch (the 'Tibet river', not to be confused with the Bhote Koshi/Sun Koshi farther west) flows down from the Nangpa La on the Tibetan frontier through the Thame valley and joins near Namche Bazaar. The Inukhu (Hinku) Khola and the Lamding Khola add further flow lower down. Together these tributaries make the Dudh Koshi a major tributary of the Sun Koshi.

Because so much of the catchment lies above the snowline, the basin's hydrology is closely tied to the state of the Himalayan cryosphere. Researchers at institutions such as ICIMOD study the Dudh Koshi as a bellwether basin for how shrinking glaciers and changing snowfall may alter river flow, sediment load and water availability across the Hindu Kush Himalaya in the coming decades.

The Dudh Koshi valley is the main corridor of the Everest Base Camp trek, one of the most famous trekking routes on earth, and tourism along the river is the economic mainstay of the Khumbu. The trail from Lukla to Namche Bazaar and beyond follows the river closely, crossing it repeatedly on high suspension bridges — the dramatic Hillary Suspension Bridge near Namche being the best known. The lodges, teahouses and porter and guide work supported by this trekking traffic underpin the cash economy of Sherpa villages such as Phakding, Monjo, Jorsalle and Namche.

In the hills and lower valleys the river and its tributaries supply water for drinking, livestock and the terraced farming of barley, potatoes, buckwheat and vegetables that has sustained mountain communities for generations. Small run-of-river micro-hydropower schemes have long provided electricity to settlements off the national grid.

The river's steep gradient and large flow also make it a target for large-scale hydropower. The flagship scheme is the Dudh Koshi (Dudhkoshi) Storage Hydroelectric Project, a planned reservoir-based plant straddling the Solukhumbu, Okhaldhunga and Khotang districts. Originally designed at about 635 megawatts and reported in more recent financing announcements at around 670 megawatts, it envisages a high dam and an underground powerhouse, and is being advanced by the Nepal Electricity Authority with backing from international lenders including the Asian Development Bank and the World Bank. As a storage project it is intended to firm up Nepal's dry-season power supply, but it has also drawn scrutiny over its environmental footprint.

For the Sherpa people of the Khumbu, who follow Tibetan (Vajrayana) Buddhism, the Dudh Koshi and its valley are part of a sacred landscape. The Khumbu is regarded in Sherpa tradition as a beyul — a hidden sacred valley blessed by the 8th-century master Padmasambhava (Guru Rinpoche) as a refuge — and the rivers, peaks and forests within it carry spiritual significance. Mountains such as Khumbila are revered as the abodes of protective deities, and the milky, glacier-pure waters of the river are associated with purity and renewal.

The valley is studded with Buddhist monuments — chortens (stupas), mani walls carved with the mantra Om Mani Padme Hum, and prayer flags strung above the river's gorges — and is overlooked by Tengboche Monastery, the most important Buddhist gompa in the region, set on a ridge above the confluence of the Dudh Koshi and Imja valleys. Festivals such as the masked Mani Rimdu dances at Tengboche draw both local devotees and visitors.

More broadly, as a headwater of the Koshi the river belongs to a system that has deep significance across the Hindu and Buddhist worlds of the Ganges plain downstream, where the waters that begin amid Everest's glaciers ultimately reach the sacred Ganges basin.

The upper Dudh Koshi basin lies within Sagarmatha National Park, inscribed as a UNESCO World Heritage Site in 1979 for its exceptional mountain scenery, including Mount Everest, and its glaciers, deep valleys and rare wildlife such as the snow leopard and Himalayan tahr. The park protects the headwater catchments of the Dudh Koshi and its Bhote Koshi branch, and proposals for large hydropower and other infrastructure in and near the park have raised concern among conservation bodies about impacts on its Outstanding Universal Value, habitat fragmentation and aquatic ecosystems.

The basin is one of the most glacial-lake-hazard-prone in the Himalaya. As glaciers retreat, meltwater ponds dam behind unstable moraines, and their sudden failure produces a glacial lake outburst flood (GLOF) — a violent surge of water, ice and debris down the valley. The defining event in the region was the outburst of Dig Tsho on 4 August 1985, in the Langmoche valley feeding the Bhote Koshi headwater of the Dudh Koshi system: the breach sent a wall of water and debris tearing down the valley, destroying the nearly completed Namche small hydropower plant downstream, sweeping away bridges, houses and farmland for tens of kilometres, and taking several lives. The disaster became a turning point that spurred systematic study of dangerous glacial lakes across the Himalaya.

Today, lakes such as Imja Tsho — one of the fastest-growing glacial lakes in Nepal, in the Dudh Koshi basin below Island Peak — are closely monitored, and engineering works have been carried out to lower its level and reduce outburst risk. Combined with monsoon flooding, landslides and the long-term threat of shrinking glaciers reducing dry-season flow, GLOFs make the Dudh Koshi a focus of climate-adaptation and disaster-risk efforts in the high Himalaya.