Mountain fed rivers originate in the high mountains of the Himalaya. The Kosi river is an example of this class characterized by a  large upland source area, a high ratio of upland source area to plains (5.31), and a high discharge.
The Kosi river has its source in Tibet that includes the world's highest upland it then drains a large part of Nepal before emerging onto the Gangetic plains. Its three major tributaries, the Sun Kosi, Arun and Tamur meet at one point just upstream of 10 km gorge cut through the Himalayan foothills. It emerges from this gorge at Chatra, and then flows across the Himalayan alluvial piedmont zone for some 40 km to Bhimnagar. The present main channel belt of the Kosi then follows a curved path around the western edge of the Kosi megafan, ultimately turning eastwards and forming a "yazoo" subparallel to the Ganga which it ultimately joins. The megafan of the Kosi has a radius of about 100 km downstream of Bhimnagar.