A waterway has become an unnatural black colour, leading to pollution fears among people who live on its banks.
Tony Walker, who has lived in Guisborough for over 50 years, spotted the colour change when he took his dogs for a walk to the bottom of his garden.
He has seen the beck become discoloured in previous months but the alarmingly dark shade of the usually clear water concerned him.
A spokesman for the Environment Agency, which is investigating the discolouration, said it had sent officers to the scene.