Update August 19, 2025: The Oregon Health Authority lifted its advisory warning of a harmful algal bloom on the Willamette River, stating "water monitoring has confirmed that the level of cyanotoxins in Ross Island Lagoon and the Willamette River...are below recreational guideline values for people." OHA continues to advise people be "alert to signs of cyanobacteria blooms...[which] can develop and disappear on any water body at any time when bloom conditions are favorable." 

Experts advise sensitive groups, including small children and pregnant people, to be especially careful when recreating in the river. River advocacy group Human Access Project has warned that the river remains dangerous for dogs, who are particularly susceptible to the impacts of cyanobacteria blooms, and can quickly fall ill or die from exposure. 

The change in river conditions comes after an unseasonably impressive rainstorm last weekend, which was followed by a few days of mild temperatures. Algal blooms thrive in hot weather, and the Portland area is set to heat up considerably over the next few days, so it's possible the river conditions could change yet again. As stated in a Human Access Project social media post: "Get it while you can and never take the ability to swim in our river for granted." 

The original story, published below, was first posted on August 14, 2025, with the headline "The Willamette River is Full of Green Algae." 

This summer, Portlanders have made their way down to the shores of the Willamette River in droves, keen to get the most out of our city's public swimming spot. Now, a harmful algal bloom is putting the fun on pause, at least for a while.  

The cyanobacteria bloom emerged in the Ross Island lagoon last week, and remained confined to the area for a while. But after a streak of extreme heat earlier this week, the algal bloom spread throughout most of Portland's stretch of the Willamette River. On Wednesday, August 13, the Oregon Health Authority (OHA) issued an advisory for a harmful algae bloom from Ross Island, located just south of its namesake bridge near Portland's South Waterfront, all the way north to Kelley Point Park. 

Exposure to toxins in the algae can be harmful to humans and deadly for dogs. With a few days of rain in the forecast, the current algal bloom may not stick around long (rain disturbs the water surface and helps break up algae formations). Even so, Willamette River advocates say the bloom is representative of a larger problem.

Willie Levenson, ringleader of the river advocacy group Human Access Project, is Portland's most well-known Willamette booster. While a big part of his job is talking about the fun parts of the Willamette River, Levenson has also been working overtime to get the harmful algae problem fixed. When the bloom began spreading in the Ross Island lagoon several days ago, Levenson issued a warning on social media more than a full day ahead of the OHA. A spokesperson for the agency told the Oregon Capitol Chronicle they can't issue an advisory until a state expert can provide evidence from the scene, which they apparently weren't able to do until well after the bloom was underway. 

Levenson wants to see the state act faster when it comes to notifying the public about potential health hazards in the river. But he also wants to see public officials take action to prevent the algal blooms from happening in the first place.

"Why is [a small] nonprofit having to drive the solution to this public health, water quality, and economic issue in the state's most populous city?" Levenson asked, referring to the Human Access Project. 

Harmful algal blooms proliferate in the heat, so their presence has become more common in the Willamette as Oregon summers have gotten warmer due to climate change. But there's another reason they keep occurring in Portland's river: stagnant water in the Ross Island lagoon. The lagoon is a manmade feature, created by the Army Corps of Engineers in the 1920s in order to connect three Willamette River islands. Ross Island Sand & Gravel's mining work in the area was also harmful for river ecology. The lagoon is now a prime breeding group for cyanobacteria, and the main place the blooms originate before spreading downstream. 

Levenson, with scientists at Oregon State University, are working on a plan to build a channel through the lagoon, allowing fresh water to flow in and break up the bacteria formations. But the project needs funding. Advocates hoped that would come in the recent state legislative session, but a bill to support the channel project didn't make it out of the Ways and Means Committee. According to Levenson, the team working on the project expects to achieve a milestone on the channel design work next year, which he says will "bring a lot of tangibility to the work." 

"It'll become a consensus tool, just to help demonstrate to all the different interested parties that this is something that is solvable," Levenson told the Mercury. 

And even though it's a bummer to have to put river recreation on pause, Levenson thinks the increased enthusiasm for the Willamette is crucial for achieving long-term change. 

"There's two different sides to my brain," Levenson said. "We want to get people in the river, to be connected to the river and see value in it, until the point when they can't [because of] a harmful algae bloom. Then, hopefully, people feel a sense of loss and understand why it's worth fighting for."