Lake Washington is generally safe to swim in during summer months, when public health officials actively monitor water quality at roughly 30 beaches across the lake. Most weeks, the vast majority of those beaches get clean results. Problems do come up, though, and knowing what to watch for makes the difference between a great swim and an unpleasant surprise.
How Water Quality Is Monitored
From roughly May through September, King County Public Health tests bacteria levels at popular swim beaches every week. The key organism they look for is enterococci, a type of bacteria that correlates well with swimming-related illness. If enterococci levels exceed 104 per 100 milliliters of water, the local health department issues a beach advisory or closure. Outside of swim season, bacteria at lake beaches are not regularly monitored, so you’re swimming with less information if you go in October or later.
When test results come back high, the typical cause is stormwater runoff carrying animal waste, or waterfowl congregating near a beach. Heavy rain often triggers temporary advisories. King County posts current beach status online, and signs go up at affected beaches. If you’re planning a trip to a specific beach, checking the latest advisory before you go takes about 30 seconds and is worth the effort.
Toxic Algae Blooms
Cyanobacteria (blue-green algae) blooms are an occasional concern in Lake Washington, particularly during warm, calm stretches of summer when water temperatures climb. These blooms can produce toxins that cause a range of symptoms depending on how you’re exposed. Skin contact can cause rashes and irritation. Breathing in droplets near the water’s surface can trigger coughing or a sore throat. Swallowing water during a bloom can lead to stomach pain or vomiting.
Algae blooms often look like green or blue-green scum on the water’s surface, sometimes with a foul smell. If the water looks discolored or you see floating mats of algae, stay out. Keep dogs out too, since they’re especially vulnerable to algae toxins and tend to drink lake water. When blooms are confirmed, health officials post warnings at affected beaches.
Swimmer’s Itch
A less serious but annoying risk is swimmer’s itch, a skin rash caused by microscopic parasites released by snails in the water. The parasites normally target birds and mammals, but when they encounter human skin, they burrow in and trigger an allergic reaction. The result is an itchy, bumpy rash that can last several days.
You can reduce your chances by avoiding marshy, shallow areas where snails tend to live. Towel off or shower immediately after getting out of the water. Don’t feed birds near swimming areas, since waterfowl are part of the parasite’s life cycle, and concentrating them near beaches increases the risk.
Chemical Pollutants in the Lake
Lake Washington has a complicated industrial history. PCB concentrations in sediment spiked between the 1940s and 1970s, then dropped sharply after a national ban on PCB production. Heavy metals like cadmium, chromium, lead, mercury, and nickel peaked around the mid-20th century and have been declining since. Copper and zinc remain elevated in bottom sediments, likely from ongoing sources throughout the watershed.
For swimmers, the chemical picture is mostly reassuring. The pollutants that remain are concentrated in sediment, not in the water column where you’re swimming. The practical concern is for people who eat fish from the lake. Fish consumption advisories remain in place for PCBs, PFAS, and mercury. A 2024 King County sediment study found that concentrations of many legacy chemicals continue to drop, which suggests that regulatory actions over the past few decades have been working.
Boat Traffic and Open Water
Lake Washington is a busy waterway, and boats are a real hazard for swimmers. Seattle regulations require swimmers to stay within 50 feet of shore or a dock unless they’re in a marked swimming area. If you want to swim in open water, you need a boat accompanying you within 25 feet, carrying a flotation device for you. Never swim near a boat ramp or in the Ship Canal.
Sticking to designated swimming areas solves this problem entirely. The roped-off swim zones at public beaches keep powerboats, jet skis, and sailboats at a safe distance.
Lifeguarded Beaches
Several cities around the lake staff lifeguards during summer. The 2025 season gives a good picture of what to expect. Most lifeguarded beaches open in late June and close between late August and Labor Day, though exact dates vary by city and park.
In Seattle, lifeguarded Lake Washington beaches include Madison Park, Madrona Park, Magnuson Park, Matthews Beach, Pritchard Island Beach, and Seward Park. Madrona opens earliest (late May) and several beaches stay staffed through September 1.
Bellevue runs lifeguards at six beaches, including Enatai Beach Park, Meydenbauer Bay Park, and Newcastle Beach Park, which have the longest seasons (late June through September 1). Chesterfield, Chism, and Clyde beaches have shorter windows ending in mid-August.
Kirkland staffs Houghton Beach, Juanita Beach, and Waverly Beach from late June through September 1. You can swim at unguarded beaches year-round, but you’re taking on more personal responsibility for safety. Water temperatures in Lake Washington typically peak in August and can still be cold enough in June to surprise people who aren’t acclimated.
Practical Tips for a Safe Swim
- Check advisories before you go. King County posts weekly bacteria results and any active beach closures online during swim season.
- Avoid swimming after heavy rain. Stormwater runoff temporarily raises bacteria levels, especially at beaches near creek inflows.
- Stay in designated swim areas. This keeps you away from boat traffic and in the zone most likely to be monitored.
- Skip the water if you see algae scum. Green or blue-green discoloration on the surface is reason enough to find a different beach that day.
- Towel off quickly after swimming. This helps prevent swimmer’s itch and rinses off any bacteria before it causes irritation.
- Don’t eat fish from the lake without checking advisories. Active consumption advisories for PCBs, PFAS, and mercury apply to fish caught in Lake Washington.