The Environment Agency said it believes the pollution entered the river at Wykebeck, Leeds yesterday morning and has affected more than six miles of the river, right the way to Castleford.
Fairburn Ings RSPB reserve is one of several downstream locations to be notified about the spill.
The Environment Agency said officers were investigating the source of the spill and the extent of the impact.
But they area appealing for help to trace exactly where the pollution came from.
The Environment Agency’s Mark West 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."
He said there had been no reports of any impact on wildlife or fish.
Anyone with information is asked to call the Environment Agency incident hotline on 0800 80 70 60.