It is the confluence of Rivers Ganga and Gandak. The River Gandak originates from the glaciers at Nepal-Tibet border. The River drains Nepal, passes through Royal Chitwan National Park (Nepal), Valmiki Tiger Reserve (Bihar) and finally merges in River Ganga at Sonepur. It is also known as Gandaki, Kali Gandaki or Narayani River.
The River is mentioned in ancient Sanskrit epic Mahabharata and its evolution is described in Holy Shiva Purana.
The river Gandak must have been the route of the movement of Budha and his followers from Nepal Tarai to Magadh that is why many of the stupas and similar structures, including Ashokas pillars, are found on the banks of the river.
The Temple of Lord Hariharnath is situated near the confluence of the two rivers. Another River called Mehi also meets Ganga near the confluence.