Environmental agency officers are investigating after a thick white foam was spotted covering the River Colne on Tuesday.
Pictures of the foam in the river near Lowestwood Lane in Linthwaite were posted on Facebook while the foam was also visible at Slaithwaite.
A spokesperson for the Environment Agency said: “The Environment Agency is responding to reports of a pollution incident in the River Colne near Slaithwaite, where a white foam can be seen in the watercourse.
“Our officers are on site to assess the potential impact on the river and wildlife and to investigate potential sources.
“If anyone sees any adverse effect on the river downstream, such as dead fish, please report the matter to our incident hotline on 0800 807060 so we can investigate.”
A similar incident happened on the same stretch of the river in 2012.