Sherritt International confirmed on October 31 that its Obed Mountain mine had spilled about 670 million litres of slurry including clay, mud, shale and coal particles.

It was ordered by government officials last month to clean up the river and rectify about 90,000 tonnes of sediment and toxic compounds that had made their way into the water.

The company told news service CBC Canada that the Athabasca River was safe for use by people as well as fish and wildlife.

It said in its newest test results, the plume of wastewater did erode the banks of the neighbouring Apetown Creek and caused damage.

Sediment spread for about 5km and also into a nearby forest.

However, the Athabasca River seemed to have slight or no erosion abnormalities and the trout in the waterway were able to survive the incident, it added.

The company told the CBC an investigation into the cause of the spill was underway and the sediment was being vacuumed from the creek beds.

Alberta Environment spokeswoman Jessica Potter last month said that Obed Mountain’s owners had been working with officials to remediate the spill.

In the weeks following the spill Canadian media reported that high levels of pollutants were found as far as 40km downstream.

According to Sherritt International data, its Mountain complex – which includes the active Coal Valley operation as well as the inactive Gregg River, Obed Mountain and Coleman properties – produced 3.7 million tonnes of export thermal coal last year.