The Lower Thames Valley Conservation Authority is warning people to keep their pets and children out of the Thames River in Chatham because of an algae bloom.
The cyanobacteria bloom, also known as blue-green algae, has changed the colour of portions of the river and is the same strain that has caused problems for Lake Erie.
Valerie Towsley from the conservation authority said she's never heard of an algae bloom on the river before.
"We're considering that we haven't really had too much rainfall in the last couple of months and it's been rather on the warm side, so it's had a chance to stay in one place and kind of be stagnant and soak up the rays of the sun and kind of bloom," she said.
Towsley is asking people to alert the conservation authority if they spot a fish die-off or other unusual event.