A white foamy substance has attached itself to plants growing in the River Stour.
It was spotted by a man walking his dog along the riverside path between the Stour and the St Radigund’s car park.
He feared that some kind of industrial effluent had managed to seep into the river.
But anti-litter activist Sian Pettman, who regularly dons waders to clear the river of rubbish in the Kingsmead area, believes the explanation may be far more straightforward.
“I accept it doesn’t look natural,” she said.
“But I have been told that it’s natural as a result of decaying organic matter releasing compounds that change the surface tension of the water.
“We get it every now and again on the Kingsmead stretch.”
The River Stour is home to numerous different types of wildlife including fish and birds such as ducks and moorhens.