Residents have this week reported grey sewage deposits and a strong smell of sewage at the river, close to the Kenfig Nature Reserve.
Fisherman Stuart Mears said he believed sewage had been going into the river for many months, and that it had been “absolutely devastated”.
“The river is now devoid of life. All the fish and invertebrates have died from the pumping station downwards and a lot of the bank-side creatures have left the area as well,” he said.
“This has affected around two to three miles of the river down to the sea. Local people are up in arms about it because those who use the reserve can’t go near the river now.”
Elaine Winstanley, a local councillor, raised concerns about the impact on local wildlife.
“This is an area where you’ve got kingfishers, you’ve got herons… you’ve got lots of fish, and all of those are at risk,” she said.
Local environmental charity, A Peace for Nature, described it as one of the worst sewage pollution incidents it had seen.