Avoid contact with toxic algae found in north Lake Washington

Algae bloom in Snohomish County
Photo courtesy Public Health

From Public Health - Seattle / King County

Toxic blue-green algae are accumulating in patches along the shores of Arrowhead Point in the northern part of Lake Washington. These patches (also called “scums”) are easily blown around by the wind. Therefore, Public Health-Seattle / King County recommends avoiding any Lake Washington water that appears to have patches of blue-green algae floating in it.

King County Department of Natural Resources conducts weekly tests of water collected at swim beaches of Lake Washington and other King County lakes. They also collect samples from areas of concern submitted through the State Department of Ecology’s Freshwater Algae Control Program. Public Health – Seattle / King County reviews results to assure safety for people and pets. Tests show that the algae are producing toxins, which are accumulating and drifting in some places along the lakeshore.

Avoid swallowing lake water with blue-green algae in it. People and pets should not wade or play in the lake where the scum has accumulated. Dog owners should be especially cautious not to allow animals to drink from the lake in these areas. If there is water contact for a pet, rinse their fur well to remove all algae and wash hands after.

The lake remains open to fishing, boating, stand-up paddle boarding and other recreational activities, though areas with blue-green algae should be avoided. People who wade and swim are recommended to stay away from scum patches.

Blue-green algae are actually bacteria (called cyanobacteria), which are always present in small numbers. Blue-green algae need slow-moving water, sun, and nutrients (phosphorus and nitrogen) to grow. These algae usually grow in late summer, but continuous sunny weather and an abundance of nutrients are promoting growth. With sunnier summers now and in the future, there may be more toxic algae blooms.

People can help prevent blooms by preventing nutrient runoff into the lake. Nutrients can come from daily activities such as car washing and fertilizing yards or gardens. 

Cyanobacterial liver toxins can cause health effects that are typically mild, but at high levels can be severe. Symptoms of illness from exposure to the liver toxin are flu-like and may include weakness, abdominal pain, diarrhea, and vomiting.

Children and pets are at highest risk and can become very ill. If these symptoms occur after ingesting lake water, park users should consult a doctor promptly. Consult a veterinarian if pets show signs of illness after being exposed to lake water.

For more information on cyanobacteria, visit the Washington Department of Health toxic algae website. Report algal sightings with Department of Ecology’s website.