An oil spill has led to the pollution of a six-mile (10km) stretch of the River Aire.
The oil is thought to have entered Wyke Beck in Leeds on Thursday morning and spread as far as Castleford, the Environment Agency (EA) said.
There have been no reports of impact to wildlife or fish but nearby Fairburn Ings RSPB reserve has been notified.
The EA said its officers were investigating potential sources of the spill and has appealed for information.
Mark West, from the agency, said: "We're asking homeowners and particularly businesses who use large quantities of diesel or oil to check their on-site storage for leaks and report any significant spillages they may have had.
"We're working to identify the source of the oil as soon as possible so that we can take measures to stop the pollution".