Quebec wildlife officials are investigating what's killing tens of thousands of fish in the Yamaska River, near the southwestern city of Saint-Hyacinthe.

Residents discovered the dead fish in the river last week, with several in advanced states of decay.

Caroline Villeneuve said she and her family alerted authorities after making the nasty discovery last Thursday.

"It's not normal that the fish are dead like that. We fish here every summer," she told CTV Montreal. "We won't fish this summer, and we won't eat the fish either."

Christian Blanchette, regional coordinator of Urgence-Environnement, said scientists came to the river on Thursday and Friday to look for possible causes, including spills, sewage and construction waste. They also took samples of the dead fish.

While officials won't know what's behind the deaths until the test results come in, Blanchette put forth one possibility.

"There was nothing conclusive, except when we came to this area right now. Where you can see there's no water in the river," he said, pointing out one section where the water had dried up.

Blanchette said a combination of high temperatures, low water levels and a lack of rain can lead to extra organic matter in the water. This organic matter requires oxygen to break down, which results in less oxygen for the fish. And without enough water, the fish can become trapped in dry areas and die.

"There's not much to do then (except) wait for the rain right now, if it is that, which we think it is," he said.

Blanchette said the city's drinking water is being tested every two hours to make sure it is safe. As well, the city is expected to decide on Monday if there needs to be any water usage restrictions put in place.

With a report from CTV Montreal's Angela MacKenzie