Punpun River originates on the Chota Nagpur Plateau, at an elevation of 300 metres. The river mostly flows in a north-east direction and joins the Ganga River at Fatuha which is 25 kilometres downstream from Patna. As Punpun is the tributary of Ganga, it has three main distributaries like the Butane River, the Madar River and the Mohar River.
Though the main river is Ganga, this 200 kilometers long river is mostly rain fed and carries little water in the dry season. However, during rains, the Punpun often causes heavy flood damages east of Patna district. The main catchments area of the Punpun is 8,530 square kilometers. It covers the agricultural area in the Punpun basin is about 5,000 square kilometers. The average annual rainfall for the basin is 1,181 millimeters.
Punpun River has its religious fervor. This river is mentioned in the Vayu Purana and the Padma Purana in connection with Gaya Mahatmya as the punya-punya, from which the name Pun-Pun came. It is the colloquial form. The river might have been called by this name because it was frequently in spate. The Puranas interpret the word punah-punah in a spiritual sense that sins are removed again and again by offering oblations to forefathers in the river. It symbols one of the most signified rivers for Hindus after Ganga and Phalgu River. It is the duty for every Hindus, who come to Gaya to shave his head on its banks and bathe in its waters on his way to the holy city. It is believed that Lord Vishnu in his fourth incarnation as Lord Vamana put his foot in second stride at confluence of Punpun and Ganga in Fatuha, known as Triveni. In this confluence, the mythological river Saraswati River is understood to be present, while measuring entire universe in gift from the great demon king Mahabali. To commemorate this event, there is a grand rural fair, known as Varuni Mela, is organised annually on the twelfth day of the bright fortnight of Bhadrapada of Vikram calendar.
Incidentally, the same day is celebrated as Onam, a South Indian festival in Kerala. The Keralite worships arrival of King Mahabali, while in Fatuha incarnation of Lord Vamana is celebrated which culminates in worship of Lord Ananta which means the endless. This is another name of Lord Vishnu on the third day since beginning of the celebrations.