Quebec City dumped approximately 46 million litres of sewage into the St. Lawrence River overnight as part of planned maintenance work.
City officials said the 12-hour dump was an unavoidable part of doing maintenance necessary to prevent major equipment failures.
The city said it planned the work in order to limit its duration.
City technicians will inspect the riverbank in order to determine how much pollution washed ashore.
Quebec's wastewater treatment system processes 137 billion litres of sewage annually.
Municipal authorities said they have taken all possible measures to reduce the environmental impact of the operation: the spill took place in dry weather, during the minimum flow periods of wastewater (at night), and using holding tanks as much as possible.
The Montreal administration of Mayor Denis Coderre was criticized in 2015 for the planned spill of 4.9 billion litres of wastewater into the river during repairs to a huge interceptor.
It was initially estimated that 8 billion litres would be dumped directly into the river, but the repair work was completed three days earlier than planned.
An environmental assessment of that dump found that after four to ten days, there was no trace of any pollution.
With a file from The Canadian Press
