The Environment Agency are investigating after a large amount of white foam appeared on the River Irwell.
Many people have shared photographs on social media of the unknown substance covering the river between Salford and Manchester city centre.
This video was filmed near Trinity Way in Salford by near Trinity Way by Cathy Hurman:
The Mersey Rivers Trust says there are as yet no reports of any wildlife in distress, though they added "lets hope it stays that way."
It is unclear what the substance is at this stage, or how it came to be in the river.