The incident happened last evening at Ganga barrage here, they said.
The body of the girl, identified as Seema, is yet to be recovered. Divers from the police department launched a look-out for the girl, but she was not found due to the heavy flow of water in the Ganga, they said.
Seema and her lover were loitering near Ganga barrage for a long time. According to an eye-witness, the duo had an argument after which the girl jumped into the river from the barrage, police said.