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

Summary

A study by King County scientists – recently published in a national scientific journal – found surprising improvements in Lake Washington’s water quality this century, even as the watershed’s population increased by 350,000 people.

News

A landmark study by King County scientists found that Lake Washington has become clearer this century even as the surrounding population rapidly increased, demonstrating that cities can grow without harming urban lakes.

The study results – recently published in a national scientific journal – found surprising improvements in Lake Washington’s water quality from 1998 through 2022, even as the watershed’s population grew by more than 350,000 people. During those 25 years, spring algae blooms decreased by 50% while summer underwater visibility increased by 3 feet.

Experts believe that multiple environmental protections likely contributed to the meaningful progress, including stormwater upgrades, erosion controls, transitioning homes from septic systems to the regional sewer system, and many everyday personal actions in the Lake Washington watershed where nearly 20% of the state’s population lives.

“Thank you to the cities, community groups, residents and staff for their leadership and work protecting the water quality of one of the nation’s premier urban lakes,” said King County Executive Girmay Zahilay. “Through sustained investments and improvements to our built environment, King County has achieved a scientific milestone. I look forward to building on this progress and protecting Lake Washington’s water quality so that everyone who lives, plays, or depends on it can enjoy it for generations to come.”

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.”

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.

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