Lord Ram after being established as the king of Ayodhya was advised to performed sraddha for his father Dasharatha at the holy place of Gaya. Lord Rama accompanied by Sita and brother Laxman came to Gaya. As per the instructions of the sage who was going to perform sraddha, Lord Ram & Laxman went out to fetch the necessary ingredients. As auspicious time for the sraddha was passing by & Lord Ram- Laxman were nowhere in sight, The sage asked sita devi to do the pinda dana herself. She was asked to make balls (instead of grains) of sand from the river phalguni (phalgu river).
Sita devi’s performance of the pinda daan bore fruits when Dashrath Maharaj’s hand appeared from underground to accept the offerings made by her. Later when Ram & laxman arrived with the ingredients, sita informed them of the entire story. Lord Ram wanted some proof for this incident. Seeing a cow standing nearby , sita devi asked her to give witness . She also asked the river Phalguni & a Banyan tree to bear witness.
When cow & the river did not gave witness, sita devi became upset & cursed both of them cow was cursed to have her mouth always unclean due to continuous drool of saliva. And river was cursed to be waterless on its river bed. (River phalguni thus became phalgu which also means False River)
The banyan tree ,however, gave witness. Sita Devi became pleased & offered it the blessings of becoming immortal & never shred its leaves throughout the year.This tree is today called as Akshay Vat.(immortal banyan)