Caroline McGough of Bispham Avenue filmed the surprising site on Wednesday afternoon, which she said lasted for around an hour
Other people have reported foam and strange colouring in the Mill Lane area of Farington and Western Drive, Leyland.
>>>Click here to read about oil pollution in the River Lostock.
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Caroline, who reported the incident to the Environment Agency, said: "It was like that for over an hour, the water water had a green/grey colour to it all day."
Alice Hazeldine said: "My husband reported it to the Environment Agency yesterday.
"Last week he saw a large number of fish! There is a kingfisher on the river which shows how clean our river was. That, whatever it is, could kill the fish. It’s absolutely unbelievable that some idiot has contaminated our beautiful river."
Alan Brierley said: "There were otters reported in the river a couple of weeks ago, this is terrible, the culprits need tracking down and prosecuting to the full extent of the law. Disgraceful."
Penwortham Councillor Keith Martin said: "It could be something that has been poured down a roadside drain. Many road drains will run to a watercourse. Tis why it is an offence to put anything down them."
Some people said the foam had the appearance of shaving foam or bubble bath.
The Environment Agency
An Environment Agency spokesperson said: "The Environment Agency would like to thank members of the public who reported concerns of a pollution on the River Lostock near Lostock Hall, Preston.
“Our team has attended site to investigate following the reports and pictures of the pollution being posted to social media.
"However due to high river flows we believe the pollution has dispersed, as such, we can also confirm no impacts to wildlife were seen. We encourage members of the public to continue to report incidents of this nature to our 24/7 incident hotline on 0800 80 70 60.”