Editing Geography of Nepal (section)

== River systems ==
{{see also|List of rivers of Nepal}}
Nepal has three categories of rivers. The largest systems -— from east to west the [[Koshi River|Koshi]], [[Gandaki River|Gandaki/Narayani]], [[Karnali River|Karnali/Goghra]] and [[Sarda River|Mahakali]]—originate in multiple [[Tributary|tributaries]] rising in or beyond the high Himalaya that maintain substantial flows from snowmelt through the hot, drought-stricken spring before the summer [[Monsoon of South Asia|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 [[alluvial fan|megafans]] exceeding {{convert|10000|km2|mi2|-3|abbr=on}} in area.

The Koshi is also called ''[[Sapta Koshi]]'' for its seven Himalayan tributaries in eastern Nepal: [[Indravati River, Nepal|Indrawati]], [[Sun Kosi|Sun Koshi]], Tama Koshi, [[Dudh Kosi|Dudh Koshi]], Liku, [[Arun River (Nepal)|Arun]], and [[Tamor River|Tamor]]. The Arun rises in Tibet some {{convert|150|km|mi|-2|sp=us}} beyond Nepal's northern border. A tributary of the Sun Koshi, [[Bhote Koshi]] also rises in Tibet and is followed by the [[Arniko Rajmarg|Arniko Highway]] connecting [[Kathmandu]] and [[Lhasa (prefecture-level city)|Lhasa]].

The ''[[Gandaki River|Gandaki/Narayani]]'' has seven Himalayan tributaries in the center of the country: [[Daraundi River|Daraundi]], [[Seti Gandaki River|Seti Gandaki]], Madi, Kali, [[Marsyandi]], Budhi, and [[Trisuli River|Trisuli]] also called ''Sapta Gandaki''. The ''Kali Gandaki'' rises on the edge of the [[Tibetan Plateau]] and flows through the semi-independent [[Mustang (kingdom)|Kingdom of Mustang]], then between the 8,000 meter [[Dhaulagiri]] and [[Annapurna]] ranges in the [[Kali Gandaki Gorge|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 [[Ghaghara River|Karnali]] drains western Nepal, with the [[Bheri River|Bheri]] and [[Seti River|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 River|Indus]] and its major tributary the [[Sutlej]], the Karnali—a [[Ganges Basin|Ganges tributary]]—and the [[Yarlung Zangbo River|Yarlung Tsangpo]]/[[Brahmaputra River|Brahmaputra]].  It is the centre of the universe according to [[Mount Meru (mythology)|traditional cosmography]]. The [[Sarda River|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 River|Mechi]], [[Kankai River|Kankai]] and [[Kamala River|Kamala]] south of the Kosi; the [[Bagmati]] that drains [[Kathmandu Valley]] between the Kosi and Gandaki systems, then the [[West Rapti River|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.

[[Image:NepalOMC.png|thumb|500px|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 [[2008 Bihar flood|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.<ref>{{cite book |last1=Aryal |first1=Ravi Sharma |last2=Rajkarnikar |first2=Gautam |year=2011 |title=Water Resources of Nepal in the Context of Climate Change |page=vii |url=http://www.wecs.gov.np/pdf/water-recource-climate-change.pdf |location=Kathmandu |publisher=Government of Nepal, Water and Energy Commission Secretariat |access-date=Dec 9, 2013 |archive-url=https://web.archive.org/web/20160103110505/http://www.wecs.gov.np/pdf/water-recource-climate-change.pdf |archive-date=January 3, 2016 |url-status=dead }}</ref>

Since uplift and erosion are more or less in equilibrium in the Himalaya, at least where the climate is humid,<ref>{{cite journal |last=Hack |first=John T. |year=1960 |title=Interpretation of Erosional Topography in Humid Temperate Regions |url=http://earth.geology.yale.edu/~ajs/1960/ajs_258A_11.pdf/80.pdf |journal=American Journal of Science |volume=258-A |pages=80–97 |access-date=December 10, 2013 |archive-date=17 April 2012 |archive-url=https://web.archive.org/web/20120417030202/http://earth.geology.yale.edu/~ajs/1960/ajs_258A_11.pdf/80.pdf |url-status=live }}</ref> 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 fan]]s 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.<ref>
{{cite journal |last1=Devkota |first1=Lochan |last2=Crosato |first2=Alessandra | last3=Giri |first3=Sanjay
|year=2012 |title=Effect of the barrage and embankments on flooding and channel avulsion, case study Koshi River, Nepal
|url= http://resolver.tudelft.nl/uuid:b9026f10-e9b8-4cc6-9e27-93416804eac5 |journal=Rural Infrastructure |volume=3 |issue=3
|pages=124–132 |access-date=December 9, 2013}}</ref> Traditional ''[[Mithila (Nepal)|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.<ref>{{cite web |url=http://onesstandpoint.blogspot.com/2009/05/floods-of-mithila-region-raising.html |title=Floods of Mithila Region: Raising Questions on Survival |last1=Thakur |first1=Atul Kumar |date=May 7, 2009 |website=Standpoint |access-date=December 9, 2013 |archive-date=18 December 2013 |archive-url=https://web.archive.org/web/20131218165541/http://onesstandpoint.blogspot.com/2009/05/floods-of-mithila-region-raising.html |url-status=live }}</ref>

Large [[reservoir]]s 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 [[hydroelectricity|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 well]]s that in the aggregate are unsustainably "mining" groundwater.
<ref>{{cite news |last=Brown |first=Lester R. |date=November 29, 2013 |title=India's dangerous 'food bubble' |url=https://www.latimes.com/opinion/la-xpm-2013-nov-29-la-oe-brown-india-food-bubble-famine-20131129-story.html |newspaper=Los Angeles Times |archive-url=https://web.archive.org/web/20131218120818/http://articles.latimes.com/2013/nov/29/opinion/la-oe-brown-india-food-bubble-famine-20131129 |archive-date=December 18, 2013 |url-status=live |access-date=Dec 10, 2013 }} [http://www.earth-policy.org/plan_b_updates/2013/update119 Alt URL] {{Webarchive|url=https://web.archive.org/web/20140716002335/http://www.earth-policy.org/plan_b_updates/2013/update119 |date=16 July 2014 }}</ref>

Depletion of aquifers without building upstream dams as a sustainable alternative water source could precipitate a [[Malthusian catastrophe]] in India's [[Food security|food insecure]] states [[Uttar Pradesh]]{{citation needed|date=November 2019}} and [[Bihar]],<ref>{{cite book |author=The United Nations. World Food Programme |title=Food Security Atlas of Rural Bihar |url=http://www.indiaenvironmentportal.org.in/files/FSARB.pdf |location=New Delhi |publisher=Institute for Human Development |year=2009 |access-date=Dec 11, 2013 |archive-date=14 December 2013 |archive-url=https://web.archive.org/web/20131214121843/http://www.indiaenvironmentportal.org.in/files/FSARB.pdf |url-status=live }}</ref> with over 300 million combined population. With India already experiencing a [[Naxalite–Maoist insurgency]]<ref>{{cite web |url=http://www.geopoliticalmonitor.com/the-naxalite-insurgency-in-india-3823/ |title=The Naxalite Insurgency in India |last1=Kennedy |first1=Kristian A. |date=May 17, 2010 |website=Geopolitical Monitor |access-date=December 11, 2013 |archive-date=15 December 2013 |archive-url=https://web.archive.org/web/20131215140334/http://www.geopoliticalmonitor.com/the-naxalite-insurgency-in-india-3823/ |url-status=live }}</ref> in Bihar, [[Jharkhand]] and [[Andhra Pradesh]], Nepalese reluctance to agree to water projects could even seem an existential threat to India.<ref name="Malhotra">{{cite journal |last=Malhotra |first=Pia |date=July 2010 |title=Water Issues between Nepal, India & Bangladesh, a Review of Literature |url=http://www.ipcs.org/pdf_file/issue/SR95.pdf |journal=IPCS Special Report No. 95 |publisher=Institute of Peace and Conflict Studies |location=New Delhi |page=11 |access-date=December 11, 2013 |archive-date=26 November 2013 |archive-url=https://web.archive.org/web/20131126095018/http://www.ipcs.org/pdf_file/issue/SR95.pdf |url-status=live }}</ref>

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.<ref>{{cite journal |last=Thapa |first=A.B. |date=January 2010 |title=Revision of the West Seti Dam Design in Nepal |url=http://www.nepjol.info/index.php/HN/article/download/4195/3569 |journal=Hydro Nepal |place=Kathmandu |issue=6 |access-date=December 11, 2013 |archive-date=14 December 2013 |archive-url=https://web.archive.org/web/20131214021551/http://www.nepjol.info/index.php/HN/article/download/4195/3569 |url-status=live }}</ref> Second, global warming has led to the formation of [[glacial lake]]s dammed by unstable [[moraine]]s. Sudden failures of these moraines can cause [[Glacial lake outburst flood|floods]] with cascading failures of manmade structures downstream.<ref>{{cite journal |last=ICIMOD |year=2011 |title=Glacial Lakes and Glacial Lake Outburst Floods in Nepal |url=http://www.icimod.org/dvds/201104_GLOF/reports/final_report.pdf |publisher=International Center for Integrated Mountain Development |place=Kathmandu |access-date=December 11, 2013 |archive-date=24 January 2014 |archive-url=https://web.archive.org/web/20140124041219/http://www.icimod.org/dvds/201104_GLOF/reports/final_report.pdf |url-status=dead }}</ref>

Third, sedimentation rates in the Himalaya are extremely high, leading to rapid loss of storage capacity as sediments accumulate behind dams.<ref>{{cite journal
 |last=Choden
 |first=Sonam
 |year=2009
 |title=Sediment Transport Studies in Punatsangchu River, Bhutan
 |url=http://lup.lub.lu.se/luur/download?func=downloadFile&recordOId=1415878&fileOId=1415879
 |publisher=Lund University, Water Resources Engineering
 |place=Lund, Sweden
 |access-date=December 11, 2013
 |archive-date=15 December 2013
 |archive-url=https://web.archive.org/web/20131215211600/http://lup.lub.lu.se/luur/download?func=downloadFile&recordOId=1415878&fileOId=1415879
 |url-status=live
 }}</ref> 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.<ref name="Malhotra" />