The police took swift action and arrested three persons in connection with the incident.
Police officials said the girl, Sobha Kumari had been missing since June 22 and a complaint had been lodged in this regard.
"Prima facie it seems that Kumari was killed somewhere else and the culprits tried to throw the body in the river so that it could not be identified," said Gaya ASP Ashok Kumar.
The ASP said love affair of the girl's mother could be the motif behind her disappearance and murder.
"Kumari had gained knowledge about her mother's affair with the youths. They killed her to ensure her silence. We are probing the case," the ASP said.