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Zion National Park Just Found Toxic Bacteria on Two of Its Most Popular Hikes

The National Park Service has advised hikers to avoid direct exposure to water colonized by cyanobacteria.

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Zion National Park officials have told visitors to exercise caution on some of its most popular hikes following the discovery of toxic bacteria in the water there, the park says.

On Friday, the park issued a danger advisory, warning visitors to avoid primary contact with water from North Creek, which flows into the Subway, due to cyanobacteria contamination. The park has advised visitors against swimming and submerging themselves in the contaminated water. 

At this time, La Verkin Creek and the North Fork of the Virgin River, including the popular Narrows hike, are under cyanobacteria-related advisories as well. (The Narrows are currently closed due to high flows.)

Cyanobacteria are small, aquatic, unicellular life forms capable of photosynthesis; they often grow in large colonies. While scientists once believed that cyanobacteria could only be found in contained water sources like lakes and ponds, recent incidents demonstrate that they’re also present in rivers. 

Cyanobacteria may be toxic to humans; children are particularly vulnerable to them, and they can be lethal to dogs. In addition to avoiding primary contact with the water, park officials are encouraging visitors to keep their pets leashed and out of the water, to bring their own purified water to affected areas, and to contact poison control when exhibiting signs or symptoms of cyanotoxin poisoning, such as a skin rash, drowsiness, tingling, burning, numbness, incoherent speech, seizures, vomiting, and diarrhea. 

This isn’t the first time that Zion National Park has wrestled with the microorganism, which the park originally detected in 2020. It’s possible that it’s even spreading: The extreme heat that Zion experiences, in addition to low water levels, creates an ideal breeding environment for the bacteria

“When you undertake the Narrows hike, you are committing to wading through water affected by this cyanobacteria, the park wrote on its Facebook page after detecting the bacteria in 2020. “There is currently an elevated risk to entering the water. The river is not closed, but we recommend avoiding this risk altogether by simply not entering the water.”

The current advisory is just one among several from this year. The park issued warnings in early October, in early and late June, and in May. Researchers in the area have collected samples that are as heavily contaminated as 550 micrograms per liter—six times the level commonly considered dangerous—in the park’s river system. 

Although officials are warning visitors about the threats of hiking and wading in some water sources, the affected hikes are not currently closed. Technical canyoneering is also allowed with the appropriate permits. 

Zion National Park provides monthly updates about potential health hazards here. If you’re exhibiting symptoms of cyanotoxin exposure, officials urge you to contact the Utah poison control center at 1-800-222-1222. 

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