A mixture of sand, rocks and oil is being blamed for a sewer overflow last weekend that prompted the dramatic rescue of a driver from an underpass on Victoria Avenue.

Michel Therrien, a spokesperson for the Côte-des-Neiges-Notre-Dame-de-Grâce borough, said it's normal to find gravel and sand in sewers, but not in such amounts — and not mixed with oil.

Therrien said officials are now investigating how it all got there and plugged up the sewer.

"There was some oil in the sewer, it's a fact, and we will see what [caused] that presence," he said.

Therrien said officials are looking into whether the oil came from one of the garages in the neighbourhood.

The sewer in question is cleared out every summer, he said, but its 2017 cleaning had yet to happen.

"This sewer was about to be done," he said.

Therrien says workers have since removed any remaining debris clogging the sewer.

A flash flood last weekend left a car trapped in the underpass in around two metres of water.

The trapped driver only escaped after Montreal police Const. Rafael Beaulieu dove into the water, smashed the car's window and pulled her from the vehicle.

The underpass was completely rebuilt 10 years ago and the pipes underneath were in good condition, city officials said. 

With files from CBC's Patrick Butler