Dog owners have been advised to keep their pets out of a river after a suspected pollution incident killed hundreds of fish.
Staff at Cuerden Valley Park said they received reports from visitors of dead fish floating in the River Lostock on August 23.
Further investigations revealed there was foam in the water and the river had turned "very brown" in the Whittle Area.
The Environment Agency was called out to investigate the 10km stretch and says it plans to take legal action.
Park manager Simon Thorpe said: "Visitors to the park told us they had seen dead fish and brown foamy water which we immediately reported to the Environment Agency.
"We are obviously very concerned that eco-system has been damaged and as manager of this visitor attraction this is very worrying.
"We believe that hundreds of fish have been killed in this incident.
"I'm liaising with the Environment Agency to see what the cause of the contamination is and how to manage it.
"At the moment we are advising people and dogs to keep out of the river but the rest of the park is still safe to visit."
A spokesperson for the Environment Agency said: "We have responded to reports of pollution in the River Lostock at Cuerden Valley Park between Chorley and Bamber Bridge.
"Environment officers have been on site investigating the cause of the pollution, samples have been taken for analysis and the source has been identified.
"The Environment Agency are progressing with legal action.
"Approximately 10km of river have been identified as impacted. The impact of the pollution has resulted in several dead brown trout and course fish."
Bosses at the Cuerden Valley say that activities are still continuing in other areas of the park and only the river has been affected by the incident.
Members of the public can report environmental incidents to the Environment Agency on 0800 80 70 60.