Jahnvi is one among the numerous names of Ganga River. Here is the reason shy Ganga is known as Jahnvi. King Bhagirath performed intense tapas to bring Ganga down on earth to wash away the sins of his forefathers.
Finally, Bhagirath succeeded in bringing Ganga down from heaven to earth. Ganga followed the footsteps of Bhagirath. He led her down the mountains into the plains.
The feisty Ganga continued to flow with a powerful current and she flooded the hermitage of Sage Jahnu - present day Ajgaivinath Temple at Sultanganj in Bhagalpur in Bihar.
The sacred fire was extinguished and all the items of the ashram were destroyed.
Sage Jahnu became furious. He chanted a mantra and sipped some water from Ganga.
This resulted in the disappearance of Ganga.
A worried King Bhagirath explained to the sage about the need for Ganga to flow on earth.
Sage Jahnu was moved by the plight of King Bhagirath and he allowed the river to flow from his left ear.
After this incident, Ganga was known as Jahnvi, the daughter of Sage Jahnu.