Environment experts in Quebec on Saturday were monitoring a spill that turned several kilometres of the Saguenay River red.
An unknown quantity of red aluminum production residue spilled into the river on Friday after a pipe ruptured at an Alcan plant in Jonquiere, north of Quebec City.
There is no danger to people and likely not to aquatic life either, said company spokeswoman Renée Larouche.
"We want to first reassure the population that despite the way the river looks, there is no danger to the population," she said.
Larouche said the mud is composed of 40 per cent inert solid material, and the rest is a "chemical liquid that is low-level caustic."
She said the leak was sealed within four hours, and the strong flow of the river diluted the residue's presence in the water.
River 'not nice to look at'
Alcan spent Friday evaluating measures to reduce the spill's impact, but no cleanup plan was announced.
Saguenay Mayor Jean Tremblay said he was not convinced the muddy waters will prove harmless.
"Alcan says it is not toxic, but there is no way a fish could live in that," he said. "I'm not a specialist, but I can tell you the river is not nice to look at right now."
Tremblay demanded a meeting with Alcan and the Quebec Environment Ministry to learnwhy the accident happened and how any damage to the riverbed or shores will be repaired.
Quebec Environment Ministry inspectors are investigating.
Nancy Bourgeois of the Chicoutimi Environment Committee, a local conservation group, said her office received dozens of calls Friday from alarmed residents.
She said the affected section of the Saguenay is a winter feeding area for sea trout and a breeding ground for smelt.
"What we want is answers," Bourgeois said. "It's just shocking that in 2007, this kind of thing could happen."