The images, posted in a local Facebook group, show a low-flowing section of river that several commenters identified as being around Penrhiwceiber and Abercynon. The water appears grey-green with a light surface film and patches of brownish-blue colouring.
Residents across the Cynon Valley have voiced concern after photographs shared on social media appeared to show a stretch of river with unusually discoloured water.
The post prompted dozens of comments from residents expressing concern about the river’s condition and calling for it to be investigated.
Among the issues raised were concerns about children swimming in the river, possible impacts on wildlife, and whether the discolouration could indicate pollution. One commenter also claimed their dog had experienced recurring skin problems after entering the water, although this has not been independently verified.
Several residents called for Natural Resources Wales (NRW) to carry out water quality testing.
Pollution or Natural Phenomenon?
Some social media users suggested the appearance could be linked to pollution, while others pointed out that prolonged warm weather and low river levels can encourage blooms of blue-green algae (cyanobacteria), which can produce surface scums and discoloured water.
Without laboratory testing, it is not possible to determine the cause of the discolouration from photographs alone.
According to Natural Resources Wales, blue-green algae can produce toxins that may be harmful to people and animals. Members of the public are advised to avoid contact with water where algae or unusual scums are visible and to keep pets away.
NRW asks anyone who suspects pollution to report it via its 24-hour incident hotline on 0300 065 3000.
Image of River Cynon by Adam Galt https://www.facebook.com/photo?fbid=122133384825153522&set=pcb.10165174970137044
https://corporate.dwrcymru.com/en/community/environment/storm-overflow-map
We have contacted Natural Resources Wales and Dŵr Cymru Welsh Water for comment and will update this article when responses are received.
Dwr Cymru gave this response today, 21 July 2026: If it’s naturally occurring algae as the article suggests, then we have nothing to add.
NRW has responded: A spokesperson from Natural Resources Wales said:
“We have not received any reports relating to discolouration in the River Cynon or its tributaries in the Penrhiwceiber and Abercynon area.
“Water discolouration can occur for a variety of reasons, including pollution incidents or natural phenomena such as algal growth. It is not possible to determine the cause without further investigation.
“Anyone who suspects pollution, or blue-green algae in the River Cynon or any other waterbody, should report it to us via our 24-hour incident hotline on 0300 065 3000 or online at naturalresources.wales/reportit so that it can be assessed.
“As a precaution, people and pets should avoid contact with water where there is visible discolouration, scum or algal growth.”
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