Geography of Nepal - River System

River System

Nepal has three categories of rivers. The largest systems -— from east to west Koshi, Gandaki/Narayani, Karnali/Goghra and Mahakali -- originate in multiple tributaries rising in or beyond the high Himalaya that maintain substantial flows from snowmelt through the hot, droughty spring before the summer monsoon. These tributaries cross the highest mountains in deep gorges, flow south through the Middle Hills, then join in candelabra-like configuration before crossing the Mahabharat Range and emerging onto the plains where they have deposited megafans exceeding 10,000 km2 (3,861 sq mi) area.

The Koshi is also called Sapta Koshi for its seven Himalayan tributaries in eastern Nepal: Indrawati, Sun Koshi, Tama Koshi, Dudh Koshi, Liku, Arun, and Tamur. The Arun rises in Tibet some 150 km. beyond Nepal's northern border. A tributary of the Sun Koshi, Bhote Koshi also rises in Tibet and is followed by the Arniko Highway connecting Kathmandu and Lhasa.

The Gandaki/Narayani has seven Himalayan tributaries in the center of the country: Daraudi, Seti Gandaki, Madi, Kali, Marsyandi, Budhi, and Trisuli also called Sapta Gandaki. The Kali Gandaki rises on the edge of the Tibetan Plateau and flows through the semi-independent Kingdom of Mustang, then between the 8,000-meter (26,247 ft) Dhaulagiri and Annapurna ranges in the world's deepest valley. The Trisuli rises north of the international border inside Tibet. After the seven upper tributaries join, the river becomes the Narayani inside Nepal and is joined by the (East) Rapti from Chitwan Valley. Crossing into India, its name changes to Gandak.

The Karnali drains western Nepal, with the Bheri and Seti as major tributaries. The upper Bheri drains Dolpo, a remote valley beyond the Dhaulagiri Himalaya with traditional Tibetan cultural affinities. The upper Karnali rises inside Tibet near sacred Lake Manasarovar and Mount Kailash. The area around these features is the hydrographic nexus of South Asia since it holds the sources of the Indus and its major tributary the Sutlej, the Karnali -- a Ganges tributary -- and the Yarlung Tsangpo/Brahmaputra. It is the center of the universe according to traditional cosmography. The Mahakali or Kali along the Nepal-India border on the west joins the Karnali in India, where the river is known as Goghra or Ghaghara.

Second category rivers rise in the Middle Hills and Mahabharat Range, from east to west the Mechi, Kankai and Kamala south of the Kosi; the Bagmati that drains Kathmandu Valley between the Kosi and Gandaki systems, then the West Rapti and the Babai between the Gandaki and Karnali systems. Without glacial sources, annual flow regimes in these rivers are more variable although limited flow persists through the dry season.

Third category rivers rise in the outermost Siwalik foothills and are mostly seasonal.

None of these river systems support significant commercial navigation. Instead, deep gorges create obstacles to establishing transport and communication networks and de-fragmenting the economy. Foot-trails are still primary transportation routes in many hill districts.