Officers from Environment Agency Wales are attending an incident north of Whitland, Carmarthenshire, where a large quantity of slurry has polluted the Nant Cwmfelinboeth, a tributary of the River Taf.
Officers were called to the site late last night after a member of the public reported the incident.
Agency teams remained at the site and are continuing to trace the source of the pollution, which is likely to have had a significant impact on water quality and fish life.
Biologists and fisheries officers from the Agency are currently undertaking a full assessment of the impact on the environment and local wildlife.
A spokesperson from Environment Agency Wales said:
"A significant quantity of slurry has entered the river and we are continuing work to find its source and to establish what impact it may have had.
"Local people should be aware that the river is likely to remain discoloured for several days and they may also notice signs of a foam like material in the water as the slurry begins to break down.
"This is one of a number of incidents that has happened in the area in recent times and once we have located the source we will take the appropriate action against whoever is responsible.”
If local people spot any further signs of pollution in the river they should report it to the Agency's incident hotline on 0800 807060.