Pollution along a 10-mile stretch of river has left it looking like a bubble bath and hundreds of fish dead.
The River Great Ouse between Brackley, in Northamptonshire, and Buckingham has been affected by the pollution, believed to be detergent.
The Environment Agency said because of its nature nothing could be done to remove or stop the foam and it would wait for it to dilute.
One Buckingham resident described it as like a “washing machine has exploded”.
Robin Heaven added that the “foam” was showing up near weirs, drifting in the air and that he saw “a few fish lying around”.
“Normally you can see the bottom of the river at this time of year as well as the fish in there, but it’s a brown sort of colour,” he said.
Another resident, who did not wish to be named, said foams were up to 5ft (1.5m) in height near a weir east of Buckingham.
The Environment Agency said its officers were collecting water samples to understand the environmental impact and urged people to keep dogs out of any foamy water and anglers not to fish.
“Indications this morning are that the pollution is continuing to move downstream,” a spokesman added.
After Buckingham, the River Great Ouse runs through Milton Keynes, Bedfordshire and Cambridgeshire before entering the sea in Norfolk.
Thames Valley Police has asked people to keep out of the river and its tributaries.