Thousands of fish have been killed in a stream sparking an investigation by Natural Resources Wales.
They include salmon, sea trout, brown trout, eels and bullheads.
The incident is believed to have been caused by a blockage in the river. This has now been removed and river levels are expected to return to normal shortly.
However, the Nant Mawr is an important spawning stream for fish and the incident is likely to have a serious effect on fish populations in the area.
Nant Mawr is a tributary of the Clywedog which feeds the River Clwyd.
NRW officers warn that as the river flows return to normal, the carcasses are likely to be washed downstream.
The fish may have been dead for a few days and people are asked not to touch the carcasses and allow them to be washed away naturally.
Emyr Jones, leading the investigation for Natural Resources Wales, said: “Fish such as trout and salmon are an important part of the ecology of our rivers and our economy.
“We are now investigating the cause of the blockage and will look at taking enforcement action against anyone who may be responsible.”
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