People are being asked to stay out of the water in the River Ryton after a number of fish were found dead in The Canch park, Worksop.
It is thought that pollution may have killed them. The Environment Agency is investigating.
Children have been known to paddle and fish in the water during the warm weather.
But Bassetlaw Council, which is responsible for the park, is advising people to keep away from the water until further notice.