The Kosi River called Koshi in Nepal (Nepali: कोशी नदी), is a transboundary river between Nepal and India and is one of the largest tributaries of the Ganga. The river, along with its tributaries, drains a total area of 69,300 km2 (26,800 sq mi) up to its confluence with the Ganga in India (29,400 km2/11,400 sq mi in China, 30,700 km2/11,900 sq mi in Nepal and 9,200 km2/3,600 sq mi in India), a part of Tibet including the Mount Everest region as well as the eastern one third of Nepal. 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. The river is joined by major tributaries, approximately 48 km (30 mi) north of the Indo-Nepal border, breaking into more than twelve distinct channels with shifting courses due to flooding.[1][2] Kamlā, Bāghmati (Kareh) and Budhi Gandak are major tributaries of Koshi in India, besides minor tributaries like Bhutahi Balān.[1][2] Over the last 250 years, the Kosi River has shifted its course over 120 kilometres (75 mi) from east to west.[3] and the unstable nature of the river is attributed to the heavy silt which it carries during the monsoon season. 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 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[4]

http://en.wikipedia.org/wiki/Koshi_River