The Kosi River ( कोसी नदी) is one of the largest tributaries of the Ganga River . Kosi river is called Koshi in Nepal and is a Tran boundary river between Nepal and India. The river basin is surrounded by the ridges separating it from the Brahmaputra in the north, the Gandaki in the west, the Mahananda in the east, and by the Ganga in the south. Kamlā, Bāghmati (Kareh) and Budhi Gandak are major tributaries of Koshi in India, besides minor tributaries like Bhutahi Balān. Over the last 250 years, the Kosi River has shifted its course over 120 kilometres from east to west. The Kosi River (The Sorrow of Bihar) is one of two major tributaries, the other river being Gandak, draining the plains of north Bihar, the most flood-prone area of India. This river is mentioned in the epic Mahabharata as Kauśiki.
The Kosi river has seven major tributaries. These tributaries encircle Mt Everest from all sides and are fed by the world’s highest glaciers. After descends from the mountains they merge and called simply the Koshi. After flowing 58 km in Nepal, it enters the north Bihar plains near Bhimnagar and after another 260 km , flows into the Ganges near Kursela. The river travels a distance of 729 km from its source to the confluence with the Ganga.
The Kosi river fan located in northeast Bihar and eastern Mithila is 180 km long and 150 km -wide alluvial cone shows evidence of lateral channel shifting exceeding 120 km during the past 250 years through more than 12 distinct channels. The river, which used to flow near Purnea in the 18th century, now flows west of Saharsa.
Two famous national parks are located in the Koshi river basin: the Sagarmatha National Park, in eastern Nepal and the Koshi Tappu Wildlife Reserve situated in Eastern Nepal. Sagarmatha National park is located in eastern Nepal is also included as a UNESCO world heritage site, was created on July 19, 1976.
The Kosi is known as the “Sorrow of Bihar” when it flows from Nepal to India, as it has caused widespread human suffering in the past through flooding and very frequent changes in course. The Kosi Barrage has been designed for a peak flood of 27,014 m³/s
Kosi Barrage, also called Bhimnagar Barrage after the name of the place where it was built between the years 1959 and 1963 straddles the Indo-Nepal border. It is an irrigation, flood control and hydropower generation project on the Kosi river
built under a bilateral agreement between Nepal and India: the entire cost of the project was borne by India. The catchment area of the river is 61,788 sq.km in Nepal at the Barrage site.

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