Warning after toxic algae found in river

Searching for solutions . . . The Opihi River, pictured looking west at dusk, is one of three South Canterbury rivers that are part of the Orari-Temuka-Opihi-Pareora (OTOP) zone water management project. PHOTO: GRETA YEOMAN

South Canterbury rivers are being monitored for toxic algae levels, and one has been identified as dangerous so far this summer.
Potentially toxic cyanobacteria have been found in the Opihi River at State Highway 1, meaning people and animals should avoid contact with the water until a health warning has been lifted.
Community and Public Health medical officer of health Dr Daniel Williams said the algal mats could produce toxins.
“Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips,” he said.
“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.”
No-one should drink the water from the river. Boiling it will not remove the toxin.
Environment Canterbury is monitoring sites and the public will be advised of any changes.


  • Appears as dark brown/black mats attached to rocks along the riverbed.
  • Algae occur naturally but can increase rapidly during warmer months.
  • Often has a strong musty smell and algal toxin concentrations can vary over short periods with changing environmental conditions.
  • 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 to treat every low-flowing river cautiously.

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