Riverside rangers in Leicester say an oil spill on the River Soar is the worst they have seen for decades as concern mounts for wildlife.
Heating oil has been discovered in the water and along the banks from Abbey Meadows to Birstall.
One bird has already died as a result of the spill.
Adrian Lane, who works as a senior riverside ranger with Leicester City Council, described the spill as "absolutely devastating".
He said it was one of the 10 worst pollution incidents he had seen in his 20 years as a riverside ranger in terms of its potential to harm wildlife.
Officials first noticed the spillage at the weekend but on Monday it became apparent that it was a large amount coming from a single source.
The Environment Agency has also been monitoring the problem.
Mr Lane said the spill had been contained where it entered the river and staff were now working to trace it back to its source.