Anderson Lake has been closed due to elevated levels of the potent nerve toxin Anatoxin-a.

Washington State Parks has posted “Danger – Keep Out of Lake” signs at access points to the park alerting visitors of the potential hazards. Recreational activities such as fishing, boating, and swimming are prohibited until the toxins dissipate and the lake is deemed safe for recreation. 

The high levels of cyanobacteria were detected in a water sample taken at the lake on Monday, April 17.

Jefferson County Public Health officials said the toxin level was 1.28 micrograms per liter, which is above the Washington state recreational criteria. 

Officials also said the bloom of cyanobacteria in the lake contains toxin-producing species of algae, and Anatoxin-a can result in illness and death in people and animals when ingested. 

Anderson Lake State Park remains open for other recreational activities such as hiking, biking, birdwatching, and horseback riding.  

Jefferson County Public Health has monitored local lakes for cyanobacteria since 2007 and Anderson Lake has had closures every year since then, including very high levels of toxin. 

During the same period of time, health officials noted that three dogs have been confirmed to have died due to exposure to Anderson Lake water. 

The lake was monitored for toxins this winter, and toxins were not detected, but starting in April, bloom activity and the toxin level have been steadily rising. 

Officials said Leland, Gibbs, Tarboo, and Crocker lakes have all been assessed for cyanobacteria activity this year and no evidence of blooms forming has been seen this season.