Mystery surrounds great swathes of white foam seen drifting down the River Irwell this morning.
Environment chiefs confirmed they are investigating the unknown substance spotted on the water's surface in Salford.
The Environment Agency (EA) said it was aware of foam pollution in the River Roch and further downstream in the Irwell.
The cause is unclear.
The mystery substance was spotted in Broughton and along the Salford-Manchester city centre border.
An EA spokesperson said: “The Environment Agency is aware of foam pollution in the River Roch and downstream in the River Irwell in Manchester.
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"We are currently on site investigating and are working with partners to resolve the incident.
"We are grateful for the public response in reporting the incident to our 24hour Incident Hotline on 0800 80 70 60.”
Salford council said the EA is leading the investigation.
A council spokesman added on Twitter: "We have received reports of white foam in the River Irwell.
"We have informed @EnvAgencyNW who are now on their way to site to investigate. We have asked to be kept informed of developments and will advise residents accordingly."
