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The Koshi River or Kośi River, along with its tributaries, drains part of Tibet including the Mount Everest region as well as the eastern one third of Nepal. The river is joined by major tributaries approximately 48 km north of the Indian-Nepalese frontier, breaking into more than twelve distinct channels with shifting courses due to flooding.^[1][2] Over the last 200 years, the Koshi River has shifted over 120 km from east to west.^[3] Flooding in India has extreme effects. India is second in the world after Bangladesh in deaths due to flooding , accounting for one fifth of global flooding deaths. The Koshi River (The Sorrow of Bihar) is one of two major tributaries draining the plains of north Bihar, the most flood-prone area of India^[4]

Formerly Kauśiki (named after sage Viśvāmitra because Viśvāmitra is said to have attained the status of Vedic ṛṣi or Rishi on its banks; Viśvāmitra was descendant of sage Kuśika and was called Kauśika in Rgveda), in Nepal and Bihar in northern India is a major tributary of the Ganges River (one major tributary of Koshi is Arun which has major part of its journey in Tibet ). This river is mentioned in the epic Mahabharata as Kauśiki. Seven Koshis join together to form the Saptakoshi River which is popularly known as the Koshi.

From Kāthmāndu, there is a road to some distance followed by trekking paths to Mt Everest, which crosses four major tributaries of Koshi. Nāmche Bāzār near Tibet border in Nepal (near southern base camp of Mt Everest) is the major tourist centre in the mountaineous part of Koshi belt. Birātnagar in Nepal, and Purnia and Katihār in India are major cities in Koshi Plains. Kamlā, Bāghmati (Kareh) and Budhi Gandak are major tributaries of Koshi in India, besides minor tributaries like Bhutahi Balān.

In Nepal the Koshi lies to the west of Kanchenjunga. It has seven major tributaries: Sun Koshi, Tama Koshi or Tamba Koshi, Dudh Koshi, Indravati, Likhu, Arun and Tamore or Tamar. Dudh Koshi joins the Sun Koshi at the Nepalese village of Harkapur. At Triveni Sun Koshi is joined by Arun and Tamar, after which the river is called Sapta Koshi. At Barāhkṣetra in Nepal, it descends from the mountains and it then called simply Koshi. These tributaries encircle Mt Everest from all sides and are fed by world's highest glaciers. Koshi has an average waterflow of 1564 cubic metres per second (55k cft per second). During peak floods, it increases to 18 times over.^{[citation needed]}

The Koshi river fan located in the northern part of India (in north east Bihar or eastern Mithila) is one of the largest alluvial cones built by any river in the world. This 180 km long and 150 km wide alluvial cone shows evidence of lateral shift exceeding 140 km during the past 250 years. The Koshi alluvial cone and its adjoining area has been studied in detail by remote sensing techniques. The data have been integrated with the available geological and geophysical information to decipher the causes responsible for the lateral shift of such a high magnitude fan. A satellite image shows the old palaeo-channels of the Koshi river with its former (prior to 1731) confluence with the Mahananda River north of Lava.^[5]

Gangetic River Dolphin , locally known as sons in Bihar, is an endangered specie (fresh water dolphin).

• Gangetic River Dolphin (Platanista gangetica)- Rapidly diminishing [3]

Formerly Koshi basin was heavily forested and hosted all types of wild fauna, but now only the mountaneous regions in Nepal have forests with wild fauna.

koshi or Milk River is known as the “Sorrow of Bihar” when it flows from Nepal to India, as it has caused widespread human suffering in the past due to flooding and very frequent change in course [4] [5] [6]. It is also the life line of the Mithila region of Bihar and forms the basis of folklore of the region.

• 
  Balanak Bonihar O Pallavi^[6] - Maithili Short Story collection on the village life of Mithila on the banks of the koshi and its tributaries.1994.Bhubaneswar. By Binod Bihari Verma.^[7]

1. ^ Koshi River. Encyclopedia. Retrieved on 2007-08-24.
2. ^ Koshi River, Bihar, India. The Geospacial Research Portal - Natural Hazard Management. Retrieved on 2007-08-24.
3. ^ The Sorrow Of Bihar : Koshi River. Nepalnews.com. Retrieved on 2007-08-24.
4. ^ GIS in Flood Hazard Mapping: a case study of Koshi River Basin, India. Retrieved on 2007-08-24.
5. ^ Abstract Agarwal, R. P. and Bhoj R. (1992) "Evolution of Koshi river fan, India : structural implications and geomorphic significance" International journal of remote sensing 13(10): pp. 1891-1901;
6. ^ [1]
7. ^ Varma, Vinoda Bihari (1994) Balanaka bonihara o Pallavi, tatha anya katha Maithili Pratibha, Bhuvanesvara

• GIS in Flood Hazard Mapping: a case study of koshi River Basin, India