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Virgin River tests positive for cyanobacteria that produces toxins

The green water of the Virgin River flows slowly past a large sandstone boulder; a large orange rock formation towers above in the distance. Photo: National Park Service

SPRINGDALE, Utah — Zion National Park is warning visitors not to swim or wade in the Virgin River after the water tested positive for a toxin caused by cyanobacteria. 

The Southwest Utah Public Health Department issued a public health warning about cyanobacteria in the Virgin River. 

According to the Environmental Protection Agency, cyanobacteria or blue-green algae often occurs in calm and nutrient-rich water. Some species of cyanobacteria produce toxins that can make people and animals sick. Algal blooms in Utah lakes forced health officials to issue several warnings to state residents in the past few years. 

Cyanobacteria at Zion National Park

Signs at Zion National Park warn visitors not to swim, submerge themselves, or allow pets to get in the water or drink it. 

In a news release, the National Park Service said it first learned about the cyanotoxin in the Virgin River because a dog died July 4, 2020, about an hour after swimming in the river’s North Fork. 

“Prior to its death, the dog exhibited symptoms consistent with possible exposure to cyanobacteria toxins — those produced by harmful algal blooms. The dog could not walk, was in pain, and was having seizures,” the statement said. 

Water samples taken from the river showed concentrations of anatoxin-a, the specific cyanotoxin involved, tested higher than 55 micrograms per liter in at least some of the samples. 

“The health threshold for primary recreation from the Utah Department of Health and DWQ [Division of Water Quality] is 15 micrograms per liter,” the statement continued. “Additionally, harmful cyanobacteria that produce anatoxin-a were identified in multiple areas of the North Fork of the Virgin River.” 

Health officials urge all visitors to the area to follow these guidelines: 

  • Do not swim in the North Fork of the Virgin River, and do not put your head underwater 
  • Do not drink the water from the river, even if using a filter or purifier 
  • Avoid any areas with visible algal scum 
  • Keep pets away from the water 
The Narrows hike, popular with many Zion visitors, requires hikers to wade through the water. The hike is still open, but park officials urge caution while following those guidelines.