The city Saint-Hyacinthe in Quebec says work related to the expansion of a water treatment plant was the likely cause of tens of thousands of fish deaths in the Yamaska River.

In a statement released on Monday, the city said the work involved a "planned overflow of waste water" in the river. Low water levels, and a high concentration of micro-organisms were also likely part of the problem, the city said.

The city said in the statement that there's "obviously been a series of bad decisions on the part of the municipal administration."

Christian Blanchette, a regional coordinator from Urgences-Environnement, told CTV Montreal he is shocked by the news. He said the city released 8.5 million litres of sewage into the river over an approximately 48-hour period around June 28. At the time, 10 per cent of the water flow in the river was sewage, he said.

Residents discovered thousands of dead fish in the river late last week, many in advanced stages of decay. Blanchette and other scientists came to the river to investigate and take samples of the water and fish.

Saint-Hyacinthe said it has launched an internal investigation to see exactly what happened, and is testing the water quality every two hours. The mayor of the city has assured the public that they are not at risk.

Blanchette said provincial and federal wildlife officials are also involved in the case.

With a report from CTV Montreal's Angela MacKenzie