People have been told to avoid the Rakahuri/Ashley River because of potentially toxic algal bloom.
The Canterbury District Health Board says animals, particularly dogs, should also be kept away from the area until its warning is lifted.
The warning comes after detaching mats of blue-green algae (benthic cyanobacteria) were discovered in the water.
Medical officer of health Dr Ramon Pink says the dark brown to black mats can produce harmful toxins.
He says exposure could cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
People with these symptoms should see their doctor immediately and pets showing illness after contact with algal mats should be taken to a vet straight away.
Meanwhile, warnings to stay out of Te Waihora/Lake Ellesmere and parts of Selwyn River because of ongoing algal blooms remain in place.