Pete Brabin reported what he believed was pollution to the Environment Agency after seeing the water turn thick brown at Doe Hey behind the Royal Bolton Hospital.
But the agency has now explained why the water changed appearance, saying there was no evidence of a significant pollution risk or threat to wildlife.
Mr Brabin, who regularly litter-picks in the area, said he was particularly concerned because the brook had very little water flowing through it.
He said: “It’s just destroyed everything, and I can’t believe the agency’s seen that; I’m wondering if they’ve been out and looked.”
Mr Brabin said he had seen similar incidents before and believed the lack of rainfall had made the situation more concerning.
He added: “But releasing it when there was no rainfall for weeks and weeks, it’s absolutely stupid.
The brook where concerns were reported to the Environment Agency. (Image: Pete Brabin)
“It’s just destroyed everything.”
Mr Brabin said he was worried about the effect the water could have on the wildlife using the brook.
He said: “Everything in that stream would have been destroyed with that pollution.
“You know, all the small fish, anything that’s in it, bugs and everything that feeds everything else – the food chain in it, would be destroyed.”
He also said animals including deer and foxes were regularly seen around the area and could drink from the water.
Mr Brabin said he checked the area regularly while carrying out litter picking and had previously reported similar concerns.
He said the brook would normally have enough flow for material to be washed away, but the dry conditions meant the brown water remained visible.
He added: “It was just trickling down there, all brown, thick brown stuff.”
The Environment Agency confirmed officers quickly investigated the report but found no evidence of significant pollution.
A spokesperson for the agency said: “Our officers quickly investigated the reports and found no evidence of a significant pollution risk or threat to the environment or local wildlife.
“Following a prolonged period of dry weather, rainfall can wash accumulated material into a watercourse, causing a temporary change in the water’s appearance with low environmental impact.
“Based on our assessment, this is the most likely explanation for the change observed.
“We will continue to monitor the situation and assess any further developments.”
More rainfall is expected this week following a string of heatwaves.
Mr Brabin said he hoped people would report environmental concerns when they saw them.
He said: “The more people reporting these issues, the better the place would be.
“It really would.”