This image of the Porter Brook was captured this morning by Twitter user Nicolas Folke.
The photo was taken at the section of the river near the Leadmill in the city centre.
An Environment Agency spokesman said: “Our officers are investigating a report of discolouration to the Porter Brook in Sheffield.
“We believe this is a consequence of river water being affected by an iron discharge from historical mine workings.
“Our environment officers are assessing the impact and take water samples.
“If anyone sees any pollution of this kind they are urged to report the matter to our incident hotline on 0800 807060 so we can investigate.”