Cyanobacteria bloom season is here: When in doubt, stay out

May 7, 2026

PORTLAND, Ore.—With summer just around the corner, Oregon agencies have begun their annual monitoring of the state's lakes, rivers and reservoirs for the presence of toxic cyanobacteria blooms.

Oregon Health Authority (OHA) and Oregon Department of Environmental Quality (DEQ) launched their seasonal freshwater surveillance activities earlier this week, as increasing temperatures raise the risk of cyanobacteria harmful algae blooms (cyanoHABs) in water bodies.

Cyanobacteria are beneficial bacteria found in all fresh water worldwide, but they can multiply into blooms under the right conditions, such as warm weather, sunlight, water temperature, nutrients and water chemistry. Many blooms are harmless, but some can produce cyanotoxins that make people and animals sick.

“When temperatures rise, many people head to their favorite water bodies to cool off, but those higher temperatures can also promote toxic cyanobacteria growth," said Linda Novitski, Ph.D., recreational waters specialist in the Environmental Public Health Section at OHA's Public Health Division. “Our goal is to inform people about the risks of exposure when recreating and reduce cases of illness due to cyanoHABs."

Exposure to cyanotoxins occurs when water is swallowed while swimming, or when people inhale water droplets during high-speed activities such as water-skiing or wakeboarding. Symptoms of exposure to cyanotoxins include:

• Diarrhea
• Cramps
• Vomiting
• Numbness
• Dizziness and fainting

 
Although cyanotoxins are not absorbed through the skin, people with sensitive skin can develop a red, raised rash when wading, playing or swimming in or around a bloom.

Children and pets are particularly sensitive to illness because of their size and activity levels. Similarly, livestock and wildlife can become ill and die after drinking from water bodies, troughs or other sources affected by blooms and potential toxins.

Dogs can get extremely ill and even die within minutes to hours of exposure to cyanotoxins by drinking the water, licking their fur or eating the toxins from floating mats or dried crust along the shore. It is very important to get a pet to a veterinarian as soon as possible if they exhibit diarrhea, vomiting, breathing problems, difficulty walking or standing, or loss of appetite.

During cyanoHABs season, DEQ and local partners collect water samples at water bodies around the state with heavy recreational use, a history of cyanobacteria blooms, or where a bloom is visible. Samples are sent to a laboratory for testing; OHA analyzes the results and decides if an advisory should be issued.

Very few freshwater bodies in Oregon are monitored for cyanotoxins. For this reason, it is important for people to carefully observe any water body they choose to recreate in before taking the plunge.

OHA recommends that everyone not swim or do high-speed boating in water that is foamy, scummy, thick like pea-green or blue-green paint, or where brownish-red mats are present. Additionally, since blooms can wash up on the shore, people should avoid areas with algal mats that are either attached, floating or stranded on the shore.

Even then, looks can be deceiving. Certain blooms grow on or near the bottom of water bodies such as lakes and rivers. While some blooms make and release toxins into the water, they don't change how the surface of the water looks, making them hard to see.

Community members looking for visual examples can find pictures of algae blooms in the Algae Bloom Photo Gallery or watch an explainer video on blooms at OHA's official YouTube channel. If you are unsure, follow OHA's guidance of “When in doubt, stay out."

Open recreational areas where blooms are confirmed can still be enjoyed for activities such as camping, hiking, biking, picnicking and bird watching. By being aware of signs of a bloom and taking appropriate precautions to reduce or eliminate exposure, people can enjoy water activities such as canoeing, kayaking, fishing, and boating as long as boat speeds do not create excessive water spray. Fish can be eaten if they are cleaned with water from a clean water source and fat, skin, and organs are removed before eating.

To learn if an advisory has been issued or lifted for a specific water body, visit the Harmful Algae Bloom website or call the OHA Public Health Division toll-free information line at 877-290-6767.

For health information or to report an illness, contact OHA at 971-673-0440. For campground or lake information, call the local management agency.

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