River Systems in Nepal नेपालका नदिहरू

 

River systems

Nepal has three categories of rivers. The largest systems -— from east to west the 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, drought-stricken 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 Lower Himalayan Range and emerging onto the plains where they have deposited megafans exceeding 10,000 km² (4,000 sq mi) in 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 Tamor. The Arun rises in Tibet some 150 kilometers (100 mi) 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: Daraundi, 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 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 centre 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 Lower Himalayan 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 supports significant commercial navigation. Instead, deep gorges create obstacles to establishing transport and communication networks and de-fragmenting the economy. Foot-trails are still the primary transportation routes in many hill districts.

Nepal's towns, villages, rivers and peaks

River management

Rivers in all three categories are capable of causing serious floods. Koshi River in the first category caused a major flood in August 2008 in Bihar state, India after breaking through a poorly maintained embankment just inside Nepal. The West Rapti in the second category is called "Gorakhpur's Sorrow" for its history of urban flooding. Third category Terai rivers are associated with flash floods.^[7]

Since uplift and erosion are more or less in equilibrium in the Himalaya, at least where the climate is humid,^[8] rapid uplift must be balanced out by annual increments of millions tonnes of sediments washing down from the mountains; then on the plains settling out of suspension on vast alluvial fans over which rivers meander and change course at least every few decades, causing some experts to question whether manmade embankments can contain the problem of flooding.^[9] Traditional Mithila culture along the lower Koshi in Nepal and Bihar celebrated the river as the giver of life for its fertile alluvial soil, yet also the taker of life through its catastrophic floods.^[10]

Large reservoirs in the Middle Hills may be able to capture peak flows and mitigate downstream flooding, to store surplus monsoon flows for dry season irrigation and to generate electricity. Water for irrigation is especially compelling because the Indian Terai is suspected to have entered a food bubble where dry season crops are dependent on water from tube wells that in the aggregate are unsustainably "mining" groundwater. ^[11]

Depletion of aquifers without building upstream dams as a sustainable alternative water source could precipitate a Malthusian catastrophe in India's food insecure states Uttar Pradesh^{[citation needed]} and Bihar,^[12] with over 300 million combined population. With India already experiencing a Naxalite–Maoist insurgency^[13] in Bihar, Jharkhand and Andhra Pradesh, Nepalese reluctance to agree to water projects could even seem an existential threat to India.^[14]

As Nepal builds barrages to divert more water for irrigation during the dry season preceding the summer monsoon, there is less for downstream users in Bangladesh and India's Bihar and Uttar Pradesh states. The best solution could be building large upstream reservoirs, to capture and store surplus flows during the summer monsoon as well as providing flood control benefits to Bangladesh and India. Then water-sharing agreements could allocate a portion of the stored water to be left to flow into India during the following dry season.

Nevertheless, building dams in Nepal is controversial for several reasons. First, the region is seismically active. Dam failures caused by earthquakes could cause tremendous death and destruction downstream, particularly on the densely populated Gangetic Plain.^[15] Second, global warming has led to the formation of glacial lakes dammed by unstable moraines. Sudden failures of these moraines can cause floods with cascading failures of manmade structures downstream.^[16]

Third, sedimentation rates in the Himalaya are extremely high, leading to rapid loss of storage capacity as sediments accumulate behind dams.^[17] Fourth, there are complicated questions of cross-border equity in how India and Nepal would share costs and benefits that have proven difficult to resolve in the context of frequent acrimony between the two countries.^[18]