A health warning has been issued for the Ashburton River at Boundary Road and at the hapua (river mouth).
The warning follows the finding of exposed and detaching mats of potentially toxic algae (benthic cyanobacteria) at these locations along the river.
Warnings also remain in place for the Ashburton River at Hills Road and State Highway 1.
People should avoid these areas and animals, particularly dogs, should not be allowed near the water until the health warning has been lifted.
Dr Ramon Pink, Canterbury Medical Officer of Health, said the algae looked like dark brown to black mats and could produce toxins harmful to people and animals.
“Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
“If you experience any of these symptoms, visit your doctor immediately, also let your doctor know if you’ve had contact with dark brown/black algal mats or water in this area,” Dr Pink said.
Pets that show signs of illness after coming into contact with algal mats should be taken to a vet immediately.
People and animals should remain out of the waterways until the warnings have been lifted.
Environment Canterbury is monitoring the sites and the public will be advised of any changes in water quality.
Facts about cyanobacteria:
Appears as dark brown/black mats attached to rocks along the riverbed.
A low cover of the algae can occur naturally but can increase rapidly during warmer months. Algal blooms are influenced by a combination of available nutrients in the water and sediments (such as nitrogen and phosphorus), a sustained period of low and stable flows, and favourable weather conditions (e.g. increased temperature, calm days).
It often has a strong musty smell and algal toxin concentrations can vary over short periods.
Although high river levels will remove the algal bloom, detached mats can accumulate along the shore and increase the risk of exposure to toxins.
If a health warning is in place avoid contact with the water.
Although district or city councils may place warning signs, these may not be seen at the numerous river access points, hence the need for people/ dog-walkers to treat every low-flowing river cautiously.