Real Talk: Is it safe to swim in Lake Union?

Lake Union is an iconic part of our local landscape and one of the easiest places for getting out on the water in Seattle. But swimming in the lake has traditionally been frowned upon due to the industrial activities that took place on the lake’s north end for several decades in the early 20th century. 

And yet, people continue to take the plunge. That prompted reader Anne Hilton to write us with a question: “There are signs all over about how toxic the water is, but I see folks swimming in (Lake Union) all the time. I work right by the lake but wouldn’t ever willingly swim in it. Are those signs old and the water’s safe now?”

We called up King County’s Water and Land Resources Division looking for answers and got connected with Tim Clark, one of the county’s official limnologists. (“Limnology,” by the way, is the study of freshwater lakes. We had no idea, either.) 

The answer to the question “Is it safe to swim in Lake Union?” is murkier than we had hoped, but here are some takeaways from Clark:

It’s all connected

In general, Lake Union water is as clean as any other freshwater in Seattle. The Montlake Cut and Ship Canal form a connection between Puget Sound, Lake Union, and Lake Washington, which is fed by the Cedar and Sammamish Rivers. “There’s a fair amount of water that’s moving through there, so it’s not a static body of water,” Clark says.

Less bacteria monitoring

That said, Lake Union isn’t monitored for bacteria levels in the same way as Lake Washington, because there are no designated swimming beaches on Lake Union. Clark warned that he would avoid swimming there during and after major rain events, when combined sewer overflows are possible. (As you may have guessed, “combined sewer overflow” is a nice way of saying “poop.”) You can check the county’s CSO overflow map to see if any of the combined pipes lining our waterways have overflowed in the past 48 hours.

Just don’t touch the bottom

For those who fear industry-related contaminants, Clark’s advice is to avoid touching the lake’s bottom because that’s where much of this pollution has settled. “If the sediments stay on the bottom and aren’t being kicked up into the water column and being ingested, then they aren’t a concern,” he says. The state ecology department, meanwhile, is continuing clean-up efforts in and around Gas Works Park. That’s where many of the warning signs that Anne mentioned can be found.

Location matters

Pollution or no pollution, swimming near Gas Works is generally a bad idea due to the shipping lanes that cut across the north end of the lake. At the south end of the lake, fresh sand was brought in when Lake Union Park opened in 2010, making it a good starting point for swimmers who are accessing the lake without a boat or board. 

Got a burning question about life in Seattle you’d like us to look into as part of our ongoing Real Talk series? Send us a note: [email protected].

By Caitlin Moran
Caitlin writes newsletters and stories for The Evergrey. She's worked as a journalist in and around Seattle since 2010 and is a proud resident of Capitol Hill's Summit Slope neighborhood.