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OK, really, is it safe to swim in the lake?

  • Aug 5
  • 4 min read

The Short Answer and the Nuance


Every summer, Lake Union calls to us — paddleboarders, swimmers, and anyone who's stood at the water's edge wondering if today's the day to jump in. But a question lingers: is it actually safe to swim here? Since the topic came up at our Floating Homes Association (FHA) lecture at the Center for Wooden Boats earlier this year, the FHA has been digging for an answer, trying to separate urban legend from environmental reality.


The short answer: somewhere between "it depends" and "swim at your own risk." The fuller answer is more nuanced and requires a bit more explanation. Depending on where, when, and how you get in the water, Lake Union can range from surprisingly clean to genuinely hazardous.


Here's what local water quality data, environmental experts, and historical context can tell us about swimming in our lake.


1. The Good News: Open Water Quality is Strong


Contrary to popular belief, the open water in Lake Union is quite clean. Because the lake is fed by Lake Washington (which receives clean water from the Cedar River and Issaquah Creek) and empties out through the Ship Canal, water flows through Lake Union surprisingly fast. On average, the water in the lake turns over about once a week. Note that this varies by time of year by days and even weeks based on freshwater flow.


Regional water monitoring teams sample surface as well as deep, open waters of Lake Union once or twice a month. Data consistently shows low concentrations of fecal bacteria and overall healthy open-water quality.


“...overall, and having lived here for 40 years and raised two kids on the houseboat, I am a proponent of swimming in Lake Union during swimmable summer temperatures…” says Dave Galvin, a professional water quality specialist who worked for King County for 37 years, and who has lived on a houseboat for 40.  He adds, “in the big picture, I agree that in general the lake water quality in the summer is quite good - surprisingly good - given that we are in the middle of a large urban area.  (But) storms cause problems, so I never let our kids jump in within 48 hours after a rare summer thunderstorm.”



2. The Caveat: Shorelines and Shallow Water Vary Greatly

While deep-water tests look good, shallow shoreline areas—where people actually like to jump in—tell a different story:


  • Localized Bacteria Spikes: Shoreline water quality is not routinely tested in the same way designated public swimming beaches are. Bacteria levels can vary dramatically over just 50 feet. 

  • Runoff & Wildlife: The main culprits for shoreline bacteria spikes are goose poop, pet waste, and surface runoff washed into shallow water from land.

  • Gray Water from Floating Over-Water Residences (FOWRs): While traditional floating homes are fully connected to the city's sewer system, many FOWRs (vessels used as primary residences) are not necessarily. While their toilet waste is contained and pumped out, "gray water"—the soapy discharge from sinks, showers, and washing machines—often flows directly into the lake, introducing detergents and chemicals into the near-shore ecosystem. Even if biodegradable, these substances aren't immediately non-toxic as they break down over time.

  • Sewer Overflows & Spills: After heavy rainstorms, combined sewer overflows (CSOs) or urban runoff can temporarily degrade water quality. Additionally, improper sewer hookups or broken sewer drains occasionally prompt city health warnings and dock signage—if you see a city warning sign posted, stay out of the water.  And honestly, you never know when someone might have discharged illegal flow into the lake.

3. Industrial History: Don't Touch the Bottom

Lake Union’s decades of early 20th-century industrial activity (particularly around Gas Works Park) left a legacy of contaminants settled in the lake bed.

As King County limnologist Tim Clark noted in a 2019 story with The Evergrey, as long as those pollutants stay trapped in the mud at the bottom and aren't ingested or stirred up into the water column, they aren't an active hazard to swimmers above. His main advice: don't disturb or touch the lake bottom, especially in northern areas near Gas Works.


4. The Hidden Risk: Docks, Marinas, and Electric Shock Drowning (ESD)

Water quality isn't the only safety factor. Swimming near docks and marinas carries a risk of Electric Shock Drowning (ESD). ESD occurs when stray electrical current leaks into freshwater from nearby AC electrical systems, paralyzing a swimmer.

  • Modern floating homes and updated marina docks are built with strict ground-fault protection systems to prevent electrical leakage.

  • However, older docks, aging shore-power setups, and unmonitored private docks can present unseen electrical risks. Swimming around active boat slips or older dock infrastructure is generally discouraged.


5. More information in progress

The Floating Homes Association has been working to improve the data collection. Dave Galvin adds, "We hope to test off-the-shelf bacteria test kits later this summer, so we might be able to offer tools for extremely local testing of bacteria levels in the future. Stay tuned for more answers as these commercial tests get evaluated by our local experts in coordination with King County’s Environmental Lab."


Swimmer’s Checklist: How to Stay Safer

If you do decide to take a dip in Lake Union, keep these simple guidelines in mind:

  1. Avoid swimming right after heavy rain: Wait 48 hours after major storms to avoid potential sewer overflows or heavy runoff.

  2. Don't touch the mud: Keep to deep water or stay off the lake bed to avoid kicking up toxic industrial sediment.

  3. Beware of docks and electrical connections: Avoid swimming close to older docks, active marina slips, or boats plugged into shore power.

  4. Watch out for boat traffic: Stay clear of busy shipping lanes, motorized vessel pathways, and heavy row/kayak corridors.

  5. Rinse off afterward: Always wash off with fresh water after swimming in open urban water bodies.


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