Juanita Beach in Kirkland remained closed, at least through the afternoon of June 11, after high levels of E. coli bacteria were found in nearby waters.

E. coli levels often rise to levels that warrant beach closures during the summer at various beaches in Lake Washington. It is used to get a sense of how much bacteria is in the water since the county doesn’t have the resources to test for all the various bacteria in the lake. When E. coli levels are high, it’s assumed other bacteria levels are elevated as well.

E. coli levels are often increased due to a variety of factors and identifying a single source is often impossible, said Debra Bouchard, a water quality planner with King County.

“There’s so many factors that are tied in to any given beach at any given time,” Bouchard said.

Contamination can come from birds defecating in the water, streams bringing in fecal matter from upstream especially after rain or from dogs and toddlers pooping in the lake. The county monitors four beaches that have creeks nearby: Juanita in Kirkland, Matthews in Seattle, Idylwood in Redmond and Gene Coulon in Renton.

E. coli outbreaks may not be tied to water temperatures, and at least one study conducted on Lake Michigan found that more sunlight can actually help kill off E. coli. Bouchard said this was seen in 2015, a hot and dry year, when E. coli levels stayed relatively low.

Lake Washington is also prone to outbreaks of what is commonly called blue-green algae, which can create toxins. These outbreaks are also susceptible to a variety of factors such as how much phosphorus is in the water and if the wind is blowing it toward shorelines where it can accumulate.

Blue-green algae is actually a bacteria called cyanobacteria and is commonly found on land and in lakes, rivers and other bodies of water. Warm temperatures, sunlight and nutrient-rich water can cause these to reproduce quickly into what the Washington state Department of Health calls blooms. Lakes can become cloudy with algae in a matter of days as it floats to the surface. It can become several inches thick near shorelines.

Most of the time these are not toxic, but some blooms can make humans, pets and birds ill. Signs of these toxic blooms include dead animals, sudden and unexplained illness or deaths of animals and skin rashes on humans. These toxins eventually break down in the water, but during a bloom they can produce nerve and liver toxins.

Samples of algae can be sent to the state Department of Ecology for free testing. The number of reports of algae blooms has been increasing, but Bouchard said it wasn’t clear if this was due to an actual increase, or more people being aware of the blooms and reporting them.

For E. coli, Bouchard said people should obey restrictions and keep dogs out of the water near swimming beaches, not feed birds, use high-quality swim diapers for toddlers and babies and to shower before and after going in the lakes.

More information on water testing and beach closures can be found at the King County Swimming Beach data webpage.