Environment Agency officers are investigating an incident in Rossendale.
Speaking earlier this week, the organisation said there had been a large fish kill at Whitewell Brook, covering a three-kilometre stretch of water.
Gordon Whitaker, environment manager for Greater Manchester, said marine life in the affected area has been "devastated".
"We are working to identify the source of the pollution, and to limit any further environmental damage," he added.
In related news, an abattoir in Wigton was fined £5,000 and ordered to pay court costs £3,127 at Carlisle Magistrates Court earlier this month after contaminating Pow Beck tributary with effluent containing blood.
Inspectors said a leak had turned the water to turn "blood red" and caused significant damage to the beck's ecosystem.
The pollution led to an increase in fungus growth and reduced oxygen levels, resulting in death and damage to marine life.
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