Mountain fed rivers originate in the high mountains of the Himalaya. The Gandak river is an example of this class characterized by a very large upland source area, a high ratio of upland source area to plains (3.33), and a high discharge.

The Gandak, known as the Krishna Gandak in its upper reaches, rises in the high region of Tibet and Nepal, where it drains a large region before emerging on the plains of the west Champaran district of northern Bihar. Here, it forms the obvious main supply system for the Gandak megafan, which trends southeast before joining the axial Ganga system.