Fish and eels were killed after slurry from a farm leaked into the Afon Bach in Tremeirchion on Friday.
Denbighshire Council has put up signs advising people not to swim or paddle in the sea, or let children on the beach after bathing water was affected.
The council has also warned dogs should not go in the sea during the temporary closure.
The authority said it would continue to monitor the water and once it had returned to an "acceptable standard" access to beaches would return to normal.
Natural Resources Wales said it was working to minimise the impact of the leak.
NRW said while the farmer had acted to fix the leak from the slurry lagoon, it would be monitoring the pollution levels over the next few days.
Nick Thomas, from NRW said: "Once we're happy that the risk of further pollution has passed, our next steps will be to consider enforcement action against those responsible if appropriate.
"We're taking the issue of agricultural pollution very seriously at the highest level and continue to work with the farming industry and its unions to deliver a step-change in reducing pollution incidents."
Parents are being warned not to let their children swim, paddle or play on beaches in Rhyl after a slurry leak.