The girl however survived.
Tulshiram Saini, the accused, was arrested last night, said the police spokesperson inspector Sukhada Narkar.
He has been booked for attempt to murder and other offences.
A resident of Lokmanya Nagar locality, Saini frequently fought with his wife, police said. On June 29, he took his step-daughter Ekta to Valavali in Badlapur tehsil and flung her into the Ulhas river and fled from the spot.
The girl fell on a hyacinth bed and escaped drowning. She remained there all night, till some passers-by noticed her cries in the morning. Police and fire brigade personnels rescued her.