Update 8/16/23: Cyanobacteria has quickly bloomed across the entire downtown swath of the Willamette River. According to an advisory issued on August 15 by the Oregon Health Authority, the affected span runs from the Ross Island Lagoon northward (that’s downstream) to Cathedral Park. That area includes Cottonwood Bay Park, Willamette Park, and Poet’s Beach, as well as the popular swimming areas at Duckworth Dock and Cathedral Park.

The OHA emphasizes that in the affected areas, people and pets are advised to stay dry and not drink the water, because water ingestion is the primary route of exposure (not skin contact). Other activities are safe, including catch-and-release fishing, camping, hiking, biking, picnicking, bird-watching, canoeing, and kayaking. Boating is safe as long as speeds do not create excessive water spray, which can lead to inhaling cyanotoxins. OHA water sample testing will continue this week.

Now is prime time for plunging into Oregon’s rivers, lakes, and swimming holes. But you (and your dog) might want to look before you leap. The Oregon Health Authority issued advisories for cyanobacteria blooms in four bodies of water in the Portland area.

“Pets are who we’re most concerned about,” says David Farrer, a toxicologist for the Oregon Health Authority. “We’ve never had any people that we know of die from exposure to one of these blooms, but we have had a lot of animals die from these blooms, especially dogs, sometimes livestock.”

Blooms can look like blue or blue-green paint, thick like pea soup, or foamy and reddish brown, like the photos here. As of August 2, affected areas near Portland include Aarons Lake, Pete’s Slough, and Sturgeon Lake on Sauvie Island, as well as the Willamette River near Willamette Cove. Near Vancouver, WA, advisories have also been issued for Lacamas Lake and Vancouver Lake. These warnings counsel against swimming and drinking the water, even with filtering. High-speed boating is also not recommended, due to spray that leads to inhalation. Eating fish from these waters is also ill-advised.

Cyanobacteria blooms, which are often colloquially referred to as “algae” blooms aren’t really algae, according to OHA. Cyanobacteria naturally occur in bodies of fresh water worldwide, but when they bloom en masse, they sometimes produce high levels of cyanotoxins, like microcystins and cylindrospermopsin. When ingested, they can produce red rashes on the skin, as well as gastrointestinal symptoms like nausea and diarrhea for up to 72 hours. For dogs, ingesting cyanotoxins can even be deadly.

Summer and early fall are the peak seasons for cyanobacteria blooms (they happened last August, too, and every previous year since monitoring began). Sunlight and warm water temperatures help the bacteria thrive, as do nutrients from human activity like fertilizer and waste. Climate change and high population density also contribute to the increasing frequency of algae blooms. You’re more likely to find them in shallow, warm, and still bodies of water—which, Farrer notes, can also include backyard ponds or livestock troughs.

It’s impossible to tell whether a bloom is toxic just by looking at it, and not all bodies of water are regularly tested for cyanotoxins. Thus, experts say to avoid bodies of water with visible cyanobacteria blooms. You can check out a complete list of Oregon advisories here, and Washington advisories here.