An "unprecedented" spill from a mine tailings pond in Canada’s pristine British Columbia has prompted a local state of emergency and water ban while environmentalists worry the contamination could have far-reaching environmental impacts as salmon return to spawn.

“This is a massive and unique event in Canada,” Gabriella Rappel, of Sierra Club Canada, told Al Jazeera. “It’s really unprecedented in such a beautiful pristine environment that’s so important for fisheries as well as the local people.”

A mine tailings pond for Mount Polley copper-gold mine, operated by Vancouver-based Imperial Metals, breached a barrier on Monday, discharging at least 10 million cubic meters of contaminated water into nearby waterways, a press release by B.C.’s Ministry of Environment said Wednesday.

The pond contained tailings, or waste rock from the mining process, contaminated with unknown levels of arsenic, mercury, lead, copper and cadmium, among other toxins and heavy metals, according to local news reports.

“Officials with the Ministry of Energy and Mines do not recall anything of this magnitude in at least the last 40 years,” the release said. “This is a large breach and extremely rare.”

The spill prompted officials to declare a state of local emergency in the province’s Cariboo Regional District and a water ban advisory not to drink, bathe, or feed livestock with water drawn from Quesnel Lake, Polley Lake, Hazeltine Creek and Cariboo Creek.

The contaminated water and debris flowed into a local creek, expanding its width from 4 feet to 150 feet, the ministry's release said, before entering nearby Quesnel Lake — where many salmon are expected to arrive for their annual spawning in the coming weeks.

“People are having to adjust their lives by collecting water and how they do their daily routines," Gord Sterrit, executive director of Upper Fraser Fisheries Conservation Alliance (UFFCA), told Al Jazeera. "People can do that, but the fish can’t. They can’t change where they migrate, or where they spawn." UFFCA is a First Nations conservation group that provides support for 22 aboriginal bands in the area.

Sterrit and other locals from fisheries on the lake are worried the spill could impact fish for years to come. Quesnel Lake and its connected waterways are important habitats for Chinook and Sockeye Salmon, as well as Rainbow Trout and White Sturgeon — an ancient species that can live for more than 100 years and is considered "endangered" by U.S. standards or "critically imperiled" in B.C.

“This is not just about short-term effects, we’re worried about the long-term effects because if the fish are pushed out it will affect the salmon for years down the road — people rely on those fish for their winter food supply,” Sterrit said.

The spill is “on a scale that we’ve never seen before,” Craig Orr, executive director of the Watershed Watch Salmon Society, said, adding that the debris from the discharge by itself is enough to cause substantial fish mortality.

“Sockeye babies grow for at least a year in the Quesnel Lake system,” Orr said, noting that the salmon are heading to the lake to spawn now.

“Chemicals and heavy metals in the water raises concerns if the levels are high enough to kill fish. But at lower concentrations, they can still have negative effects."

Copper at low levels can impair the fish's sense of smell, a critical part of navigation.

The B.C. Ministry of Environment along with officials from Imperial Metals have carried out water testing and the results are expected Thursday afternoon.

“People are on the scene now,” Jake Jacobs, a spokesman at the Ministry of Energy and Mines, said, adding that he believed the breach had been stopped.

Imperial Metals did not reply to Al Jazeera’s request for a statement, but said in a press release on its website Tuesday that the discharge had been stabilized. A Wednesday release by the Ministry of Environment said the “flow out of the breach has decreased dramatically, but has not completely stopped.”

“Our first priority is the health and safety of our employees and neighbors, and we are relieved no loss of life or injury have been reported,” Imperial Metal's press release read. It added that the cause of the breach was unknown and that none of the monitoring instruments or on-site personnel had any indication of an impending breach.

The Ministry of Environment has called on Imperial Metals to submit a clean-up action plan by Aug. 15, according to a press release by the ministry.

Orr said the upside of the disaster may be that new government regulations are put in place, and that inspections and monitoring on such sites are made more stringent.

But Sterrit said the accident has only confirmed fears among locals about mining and other resource extraction projects and raises new fears about more planned projects in the area.

“This has the potential to wipe out a lot of salmon stocks. And we are concerned about a mine with the same design at Red Chris,” Sterrit said of a copper and gold mine Imperial Metals is constructing.

Experts said the Mount Polley spill will likely delay Red Chris for at least a year. The discharge can be compared to a similar incident in Spain in 1998, in which the investigation into the breach took eight months, Raymond Goldie, an analyst with Salman Partners, told Business Vancouver, a local news website.

“I’m sure there’ll be the same thing here, and I find it difficult to imagine Red Chris being allowed to start up before the government assured itself what the cause of Mount Polley and also assured itself that this wouldn’t happen to Red Chris,” Goldie said. “I think it could delay Red Chris by as much as a year.”

But locals question whether the government will do enough to address the disaster and prevent future spills.

“We’re going to have to wait and see what happens,” Orr said. “We need long-term monitoring because these heavy metals are lasting. Whether those effects will be monitored is doubtful.”