Niagara police say officers will be combing a roughly 30-kilometre stretch of the river's edge up from where it empties into Lake Erie, while the marine unit will search the river itself.
They say the decision doesn't stem from a tip, but is part of their ongoing search for evidence that could help identify the torso.
Police have reviewed 50 missing persons cases and say the vast majority do not match the description of the dismembered victim.
Police say the torso had been in the water for four to 10 days before it was found by a passerby last Wednesday.
Post-mortem results indicate the torso belongs to a Caucasian woman between 31 and 55 years old. The woman had a pierced navel and two caesarean section scars.
Investigators say this case is not connected to the slaying of a Toronto woman whose dismembered remains turned up last month in Mississauga and Toronto.