Authorities just found a toxin in the Jordan River. Here’s why you should stay out of the water.

The Salt Lake County Health Department warned people Friday not to enter the Jordan River after test results found an algae-related toxin in the water.

The results showed that the neurotoxin anatoxin-a was present at a "low level" of 0.1 micrograms per liter in the river, and the health department advised people and animals to avoid entering or drinking river water. The toxin could be fatal if ingested.

"Toxins present at this level have not been shown to present a health threat to people on the surface of the river, such as while kayaking or boating," said a post on the health department's Facebook page.

The samples that tested positive for the toxin were taken from the Jordan Narrows, where the Jordan River enters Salt Lake County, the post said, "but we advise caution for the entire river as algae and toxins can spread and change quickly."

The department said it would post warning signs at Wheeler Farm in Murray and Blackridge Reservoir in Herriman "due to their popularity as recreational points." Herriman said Friday evening that it had closed Blackridge "until further notice" as it tests the waters.

"We will continue to monitor and sample the Jordan River and its related canals and will let you know of any changes to our advisories," the health department wrote.

Last month, Utah County's health department issued similar warnings for parts of Utah Lake after a toxic algal bloom and elevated levels of E. coli were detected in routine water samples. In recent weeks, those warnings have expanded to the entire lake.

mnoble@sltrib.com

Twitter: @mnoblenews