How denser multifamily housing helped clean up Lake Washington

Conventional wisdom holds that as cities grow, nearby bodies of water suffer from increased pollution. According to a 25-year study by King County scientists published in the peer-reviewed journal Ecosphere last year, Lake Washington grew clearer, less nutrient-polluted, and measurably healthier between 1998 and 2022, even as the watershed added more than 350,000 residents.

According to the 2025 study, shifting growth toward denser multifamily housing helped protect the lake by upgrading older infrastructure and containing runoff.

The study arrived last year as Seattle was shaping its new 20-year comprehensive plan to encourage denser housing and to meet statewide requirements for increased middle housing development. This year, the process to complete that plan is tied up in court after a ruling iced efforts to shape policy mandating 6-story neighborhood centers along transit corridors — policy that would reshape the zoning in many of the area neighborhoods along the shore of Lake Washington like Leschi and Madison Park.

Central to the research is the long-term offshore monitoring station just off Madison Park, designated as Station 0852. Situated near the lake’s deepest basin approximately 214 feet below the surface, the station provides data that aquatic scientists consider representative of a large volume of the lake.

Across the 25 years of monitoring included in the study, researchers documented consistent ecological improvements. Surface chlorophyll concentrations dropped by about 24% per decade during the main spring bloom. That decline was accompanied by an around 8.6% per decade increase in underwater Secchi disk visibility, which measures clarity by tracking how deep a patterned disk remains visible, adding roughly three feet of clear water over the study period.

Dissolved silica concentrations in surface waters climbed, rising by 98% per decade in the spring because fewer single-celled algae called diatoms were consuming the mineral and carrying it to the lake bed as they died. In addition, peak late-winter and spring levels for total phosphorus and total nitrogen dropped by 5.6% and 4.9% per decade, while autumn dissolved oxygen near the lake bottom increased by 17% on average, indicating that less decaying organic matter was sinking and stripping oxygen from the deep water.

The study points to a key driver behind the turnaround — the shifting nature of how the region builds and develops.

Between 2000 and 2020, the watershed population grew by 34%, but developed land area expanded by only 4.7%, and regional forest cover held steady at roughly 43% of the basin. Rather than clear-cutting to accommodate housing, urban centers around the lake accommodated growth by converting lower-density areas into higher-density multifamily developments.

The study notes that redevelopment projects typically replaced older, pre-1990 properties with modern construction that triggered updated stormwater filtration, strict site-erosion controls, and connections from older private septic systems into municipal sewer lines. Combined with statewide bans on phosphorus in laundry detergents and lawn fertilizers, infill housing effectively reduced the per-person nutrient runoff reaching the basin.

The study did not directly track nearshore fecal bacteria levels, such as E. coli or fecal coliforms behind the spikes in bacteria that periodically lead public health officials to close swimming beaches during warm summer months.

For the shoreline communities of District 3 including Madison Park, Madrona, Leschi, and the Washington Park Arboretum, the findings provide local insights.

Total alkalinity, a measure of dissolved mineral salts and carbonates that can alter water chemistry and had risen noticeably during previous periods of heavy land clearing, leveled off during the study’s 25-year period, removing a chemical condition known to favor nuisance blue-green algae. Sediment core analysis also revealed that toxic microcystin, a poison produced by cyanobacteria, has remained below detection limits for decades, meaning that toxic algae blooms have remained rare even as water temperatures climb.

For the neighborhoods bordering Madison Park and the rest of the city, the study demonstrates how population expansion alongside dense development can coexist with a healthy Lake Washington.

But there are limits.

The study’s handling of climate change represents one major hole. Warmer water accelerates decay and interacts with falling nutrient levels to alter the food web toward smaller, single-celled picoplankton. Fisheries scientists warn that this shift threatens to starve grazing zooplankton and collapse food supplies for federally protected juvenile Chinook salmon, meaning the clearer water could reflect a depleted ecosystem.

And even though surface waters warmed, the lake did not begin layering into warm and cold water any earlier in the year, revealing gaps in how well climate models explain the lake’s behavior.

Another gap? Phosphorus reductions explain less than half of the 25% per decade collapse in spring algae blooms. Attributing the lake’s clarity primarily to land-use policies overlooks how rising water temperatures, shifting biological communities, and grazing pressures could suppress algae independently of human management.

Important local questions are also unanswered. The connection between apartment construction and cleaner runoff is only a broad correlation, leaving researchers without the neighborhood-level data needed to prove a direct cause.