Quebec's Environment Ministry says it did not authorize the dump of 8,000 tonnes of raw sewage by the City of Saint-Hyacinthe that experts and local officials believe led to the deaths of thousands of fish in the Yamaska River.

Ministry spokeswoman Mylène Gaudeau said the province is now investigating.

The city carried out the dump on June 28 in connection with expansion work on the city's wastewater treatment plant.

I can tell you I'm very disappointed and troubled by what happened. - Claude Corbeil, Saint-Hyacinthe mayor

Several days later, after following up on concerns from residents, provincial authorities found thousands of fish on the bottom of the river, discoloured and in an advanced state of decay.

"The inquiry will allow us to determine if any laws were broken," Gaudreau said.

City Coun. Donald Coté, who chairs Saint-Hyacinthe's waste management committee, could not confirm whether the provincial authorities were briefed on the dump.

Normally, that would be the case for such a move, he said.

Coté says the city takes full responsibility for any mistakes made that resulted in the deaths of the fish.

"We're not happy about it," he said. "We're not hiding it."

'A series of bad decisions'

A photo of the dead fish in the Yamaska River was posted to a local Facebook page on July 1. (Spotted St-Hyacinthe)

Saint-Hyacinthe Mayor Claude Corbeil said Tuesday that poor communication between municipal departments led to the dump.

Corbeil said his administration takes full responsibility for a series of errors that led to the dump being approved despite low water levels in the river and its slow flow.

"There was a lack of communication, a series of bad decisions," Corbeil said.

"I can tell you I'm very disappointed and troubled by what happened, which clearly goes contrary to our environmental values."

Corbeil also said he was "greatly troubled" that residents of Saint-Hyacinthe were not given advanced notice of the sewage release, which had been planned as far back as late April.

Drinking water quality was never at risk from the wastewater dump, the mayor said, noting the source of the city's drinking water is upstream from the wastewater plant where the dump originated.

Water levels 'not considered'

Coun. Donald Coté says Saint-Hyacinthe takes responsibility for the mistakes that led to the deaths of thousands of fish. (Kate McKenna/CBC)

Coté said the city's original plan for the sewage release was based on projected water levels in the Yamaska River.

"At that time, we were counting on a regular flow of water in the river. But the drought we're being subjected to led the river to be very low for this time of year," he said.

"Employees did not consider that low level when the flush was ready to happen."

The long weekend meant some managers were hard to reach and contributed to the situation, he added.

Corbeil said he has asked for an internal inquiry to determine why the sewage dump proceeded despite the unfavourable river conditions.

"This can't happen again," he said.

Sewage may have led to drop in oxygen

There were 651 sewage spills in the Yamaska River in 2015, according to the Rivers Foundation, a Quebec non-profit organization. (Kate McKenna/CBC News)

Sarah Dorner, an associate professor of civil, geological and mining engineering at Montreal's École Polytechnique, said the sewage dump likely led to a decrease in oxygen and an increase in the amount of ammonia in the river — which combined to kill the fish.

"These are probably the main explanations for what happened," said Dorner, the Canada Research Chair in source water protection.

While the worst is likely over, Dorner said the dump represents yet another stress on a river that already struggles with more than its share of them.

"The Yamaska has a lot of nutrient pollution from agricultural development. This river also receives raw sewage spills on a regular basis," she said.

The Rivers Foundation, a Quebec non-profit organization dedicated to protecting the province's rivers, said there were 651 sewage spills in the Yamaska River in 2015.

Montreal made international headlines in 2015 for dumping some eight billion litres of untreated wastewater into the St. Lawrence over the space of a week in order to repair parts of its sewage system. (Paul Chiasson/Canadian Press)

In 2015, the City of Montreal faced intense criticism for its decision to dump more than eight billion litres of sewage in the St. Lawrence River.