'A healthy lake sends a powerful message’: Landmark King County study of Lake Washington finds that cities can grow without harming water quality

The clearer water in Lake Washington builds on progress that King County and cities throughout the watershed made during the 1950s and ’60s when the regional wastewater treatment system was built. What had been a heavily polluted lake that was unsafe for swimming was considered by a leading University of Washington researcher to be fully recovered from sewage by the early 1970s.

As the watershed’s population grew rapidly during the ensuing decades, scientists were concerned that pollution from many small urban and suburban sources would likewise increase and harm the newly restored lake. King County’s Water and Land Resources Division began long-term monitoring of Lake Washington in the 1990s, picking up on research that the University of Washington had started in the 1960s.

“When we analyzed changes in Lake Washington’s water quality since 1998, we expected to find subtle problems and warning signals,” said Daniel Nidzgorski, an ecologist at King County’s Water and Land Resources Division and lead author of the study. “But to our surprise, we did not find any ways that population growth and development have harmed water quality. Instead, Lake Washington has had less phosphorus, less algae, and clearer water.”

Clearer water with less algae is good for swimming and other forms of recreation, though algae is the base of the lake’s food web. How these changes might affect fish and other species is a complex topic that scientists at the Water and Land Resources Division and the University of Washington are continuing to explore.

"Lake Washington is a treasured natural resource for our city, and this report is proof positive that our region can grow responsibly while still protecting our environment," said Seattle Mayor Bruce Harrell. "The progress we see in Lake Washington’s water quality reflects decades of regional commitment through smart urban planning, infrastructure, and stormwater management. Seattle will continue to collaborate with King County and surrounding cities to balance growth with our commitment to clean, thriving waterways."

Scientists identify likely contributors to the surprising development

While it’s unclear exactly what caused the surprising development, scientists have identified several likely contributors:

• Improved stormwater treatment, including retrofits to existing systems, to catch more phosphorus and prevent it from reaching streams and the lake.
• Better erosion controls at construction sites, reducing the amount of soil that washes into streams or storm drains and eventually into the lake.
• Statewide restrictions on phosphorus in lawn fertilizer as well as in laundry and dishwasher detergents.
• Residents improving their lawn care and landscaping practices and picking up more pet waste.
• Transitioning homes from septic systems to sewer systems.

These improvements stem from a mix of regulatory and voluntary actions by the county, cities, community groups, and residents. In particular, cities have played a key role in improving stormwater treatment throughout the urban and suburban areas of the watershed.

"In Kirkland, caring for our natural environment is part of who we are," said Kirkland Mayor Kelli Curtis. "This study affirms that communities can grow with grace when we intentionally balance environmental stewardship with economic investment. Kirkland appreciates King County’s leadership in using data to guide how we care for our waterways, and the results for Lake Washington speak for themselves."

The study also suggests that increasing urban density may have been beneficial for Lake Washington’s water quality. From 2000 to 2020, the watershed population grew by 34% while developed land area grew by less than 5% and forest cover remained steady. Instead of clearing intact forest to accommodate growth, the population grew in place by converting from lower-density to higher-density housing. This redevelopment likely prompted many stormwater upgrades, transitions from septic to sewer, and other changes that helped reduce phosphorus pollution.

“Does concentrating growth sacrifice urban areas to protect surrounding landscapes? Not necessarily, according to this study. Done right, we can have growth and a healthy lake,” said Josh Latterell, who leads King County’s Science Section at the Department of Natural Resources and Parks. “I’m grateful to have a team providing the useful science like this so our leaders can make informed decisions.”

The study was published in the scientific journal Ecosphere, which is managed by the Washington, D.C.-based Ecological Society of America.

Another recently published King County study based on 20 years of data found widespread improvements in the health of local streams. Scientists found that conditions are improving in both urban and rural areas with 16 of 38 watersheds improving and just one declining. A 2024 King County study of Lake Washington’s lakebed sediment shows levels of PCBs, the once common and dangerous chemical category, are expected to fall below currently detectable levels within the next two decades.

King County’s Water and Land Resources Division is one of four divisions at the Department of Natural Resources and Parks.

Contacts

Michelle Green, Water and Land Resources Division, 206-477-6151

Doug Williams, Department of Natural Resources and Parks, 206-477-4543