More than three kilometres of a Preston waterway have been affected by suspected cow slurry.
Savick Brook, which runs from the outskirts of Longridge through to Fulwood in Preston, has been affected by kilometres of cow waste.
The issue was reported to the Environment Agency this morning (Friday, November 11).
Local resident Chris Bond said: "Gallons of cattle effluent have been tipped into water on Savick Brook to the east of Preston.
"This is causing foam and a terrible smell. Poisoning the water and harming wildlife."
The EA has said that there are "no obvious harm" to wildlife.
A spokesman said: "The EA received reports of pollution in Savick Brook, Fulwood this morning.
"Environment Officers are on the scene and they are currently trying to trace the source.
"At least 3km has been affected, although no obvious harm to aquatic life has been observed.
"The pollutant appears to be cow slurry however further investigation will confirm or deny this.’
"You can report incidents of this nature to our incident hotline on 0800 80 70 60."
The lowest section of Savick Brook has been widened into the Ribble Link which connects the Lancaster Canal to the River Ribble.