Officers from Natural Resources Wales (NRW) are investigating a pollution incident on the Afon Pelenna and Afon Afan near Efail Fach in Neath Port Talbot.
The incident was reported to NRW earlier today by a local person who noticed that the rivers were discoloured and running orange.
Officers are currently on site assessing the impact and taking water samples for further analysis.
They have identified the source of the pollution and are working with those responsible to investigate the circumstances and to reduce any further damage to the area.
Huwel Manley, from Natural Resources Wales said:
“We’d like to thank those who have reported this incident to us so we can take swift action to address it.
“Our initial assessments show that the dissolved oxygen and pH levels of the river are in normal range, and we haven’t witnessed any signs of dead fish or fish in distress.
“We expect the discolouration to continue downstream of Pontrhydyfen for the next 24 – 36 hours, so we’ll be keeping a close eye on the rivers closely for any change.
“If anyone is concerned about further pollution, or notices any impacts on fish or wildlife, they should report it immediately to our incident hotline on 0800 807060.”