The Kosi River

The Kosi River—also Saptakoshi , is a trans-boundary river flowing through Nepal and India, known for its seven Himalayan tributaries. Some of the rivers of the Kosi River system, such as the Arun, the Sun Kosi and the Bhote Koshi, originate in the Tibet Autonomous Region of China. It is one of the largest tributaries of the Ganges.
Along with its tributaries, the river drains 29,400 km2 (11,400 sq mi) in China (mainly the upper Arun basin north of the Mount Everest region), 30,700 km2 (11,900 sq mi) in Nepal (the eastern third of the country), and 9,200 km2 (3,600 sq mi) in India.
The river basin is surrounded by ridges which separate it from the Yarlung Tsangpo River in the north, the Gandaki in the west and the Mahananda in the east. The river is joined by major tributaries in the Mahabharat Range approximately 48 km (30 mi) north of the Indo-Nepal border. Below the Siwaliks, the river has built up a megafan some 15,000 km2 (5,800 sq mi) in extent, breaking into more than twelve distinct channels, all with shifting courses due to flooding. Kamlā, Bāghmati (Kareh) and Budhi Gandak are major tributaries of Koshi in India, besides minor tributaries such as Bhutahi Balān.

Over the last 250 years, the Kosi River has shifted its course over 120 km (75 mi) from east to west. Its unstable nature has been attributed to the heavy silt it carries during the monsoon season and flooding in India has extreme effects. The Kosi River (The Sorrow of Bihar) is one of two major tributaries and the other river, the Gandak, drains the plains of north Bihar, India's most flood-prone area. Fishing is an important enterprise on the river but fishing resources are being depleted and youth are leaving for other areas of work.
The Koshi was also called Kausika in Rigveda, Nepal and Bihar in northern India. It is a major tributary of the Ganges. One major tributary of the Koshi is the Arun, much of whose course is in Tibet. This river is mentioned in the epic 'Mahabharata' as Kausiki. Seven Koshis join together to form the 'Saptakoshi River,' or 'Sapt Koshi,' which is popularly known as the Koshi.
Formerly known as 'Kausiki,' named after the sage Viśvāmitra, who is said to have attained the status of Vedic or 'Rishi' on the banks of the river. Viśvāmitra was a descendant of the sage Kusika. Viśvāmitra is credited with writing many well-known Vedic Hymns on the Banks of the Kosi where he had his hermitage — The Mandala 3, the Rigveda, and the Gāyatrī Mantra. The Gāyatrī Mantra is a highly revered mantra based on a Vedic Sanskrit verse from a hymn of the Rigveda (3.62.10).
The mantra is named for its vedic gāyatrī metre:  as the verse can be interpreted to invoke the deva Savitr, it is often called Sāvitrī,  its recitation is traditionally preceded by oṃ and the formula bhūr bhuvaḥ svaḥ, known as the mahāvyāhṛti ("great utterance"). The Gayatri Mantra is repeated and cited very widely in vedic literature,  and is praised in several well-known classical Hindu texts such as Manusmṛti, Harivamsa and the Bhagavad Gita.
The Koshi (Kosi in Sanskrit) is associated with many ancient spiritual stories. Kosi is mentioned in the Bal Kand section of Valmiki Ramayana as the Kausiki who is the form assumed by Satyavati after her death. Satyavati was the elder sister of Viswamitra, descendants of Kushak dynasty. In the Markendeya Purana, the Kosi is described as the primal force. Due to the violent nature of the Kosi during monsoon season, legend says that Parvati, the wife of Shiva, after defeating the demon Durg, became known as the warrior goddess Durga who transformed into Kaushiki. In Ramayana, the river Ganges is depicted as her elder sister.
According to Mahabharata epic, the God of death took the form of a woman and resides on the banks of the river to limit population growth. Kosi resonates with the folklore of Mithila. The most important depictions of Kosi folklore are Kosi as a virgin absolutely care free and full of energy and as a frustrated wife of old hermit Richeek wandering in the Himalayas. Kosi is also invoked as the mother - 'Kosi Ma'. These images capture the contradiction that is inherent in the Kosi River as a source of life and death, prosperity and destruction; a mother and an enchanting virgin.
It is also the lifeline of the 'Mithila' region, today spread over more than half of India's state of 'Bihar', and parts of adjoining Nepal. It is the subject of legend and folklore of the region; the legend of Mithila extends over many centuries. Mithila is also the name of a style of 'Hindu art' created in the area.