The greater-Seattle area is bounded by the saltwater Puget Sound and freshwater lakes including Lake Washington. In summertime, Seattle sparkles with water activities including boating, fishing and swimming.
Over the past sixty years the greater-Seattle metropolitan water treatment program has led to better water quality in Lake Washington, and yet every summer there are some “closed beach warnings” because of high bacteria levels or toxic algae in the water.
This blog article will list the reasons for water contamination in Lake Washington and will describe the water-quality monitoring efforts of the metropolitan district (King County) and the citizen activist group, Thornton Creek Alliance. You can meet & talk with the volunteers of Thornton Creek Alliance at upcoming work parties.
Sailboats make their way around Lake Washington in the summertime.
Bacterial contamination of the water
A poster from the Metro campaign of 1958. Photo #101715 of Seattle Municipal Archives.
In early Seattle raw sewage was carried via pipelines and discharged directly into bodies of water including Puget Sound and Lake Washington. By the 1950s, the population of the Seattle area had grown so much that much greater measures needed to be taken for sewage treatment.
In 1958 King County voters created Metro, a regional wastewater treatment system.
In the 1960s most of the sewer lines were diverted away from Lake Washington to treatment plants including West Point, and the South Plant in Renton. Today, Lake Washington is one of the best examples in the world of successful restoration of a lake by diversion of sewage. However, there still are some combined sewers that with overflow, can go into Lake Washington.
King County’s ongoing water testing program tests the water at the beaches of Lake Washington and sometimes “closed beach” notices are posted due to bacteria in the water.
Dangerous bacteria which could make swimmers sick, comes from goose droppings, dog waste and human fecal bacteria which has washed down in stream systems or from stormwater & sewerline overflows.
The beaches of Lake Washington are tested once per week, usually on Monday or Tuesday, and results are posted on the King County website usually by Wednesdays. No-swimming notices are posted at the beach if necessary; you can check the beach status on the King County page.
Beach bacteria testing begins in mid-May and ends in mid-September. Toxic algae testing begins in early June and ends in late October.
Phosphorus & algae
Algae growing in a pond. Photo by Eric Vance, Environmental Protection Agency.
Phosphorus is a mineral used by plants and soil microbes for normal growth. In excessive quantities, it causes the water quality problem of an algae bloom. Algae, in turn, may contain toxins. Algae is an essential part of the aquatic food web, but it becomes a problem when there is excessive phosphorus and an algae bloom occurs. The growth blocks sunlight into the water and leads to reduced oxygen levels in the water.
Sources of excessive phosphorus in Lake Washington come from decaying leaves and grass clippings, runoff from urban areas, runoff containing goose droppings, leaking sewer systems, and overflow discharges from wastewater treatment plants.
Washington State passed a law which went into effect in 2013, banning the use of lawn fertilizer containing phosphorus. This includes lawns of homes, parks, golf courses, cemeteries and schools. The law resulted in a dramatic reduction of the amount of phosphorus entering Lake Washington via surface runoff and storm drains.
The King County Environmental Lab tests the water of Lake Washington (a different test schedule from beaches) once or twice per month, to measure the levels of phosphorus and water clarity. The testing spot is at the deepest point of the lake, off of Madison Park. At the height of sewage problems in the 1950s, the water clarity was at a depth of only about thirty inches. Now, using a measuring instrument, you can see down into the lake to a depth of six feet or more. Due to reduced phosphorus, there is less algae growth in the lake. Less algae means more sunlight and oxygen in the water, for the health of fish and aquatic plants in Lake Washington.
Keeping the water clean
What can be done to help keep Lake Washington clean?
The metropolitan Seattle area has ongoing efforts to improve stormwater systems by replacing old pipes.
While we are seldom enthusiastic about big new buildings, including apartment complexes which replace private homes, their new sewer and stormwater infrastructure is actually better.
Although apartment-dwellers do own dogs, they must use the dedicated zone for their dog and must scoop the poop.
Scooping dog poop is one of the most helpful ways to prevent bacteria from washing down into our watershed and into Lake Washington. Thornton Creek Alliance supports Seattle Public Utilities’ Mitts for Mutts program by keeping dispensers stocked with scoop bags at several Thornton Creek Natural Areas, at points where people walk their dogs along the stream.
To help the environment, homeowners can have their side-sewer line scoped to look for any leaks, as these leaks are another source of contamination in the water system.
Matthews Beach at NE 93rd Street is the outlet of Thornton Creek into Lake Washington.
People should not feed ducks or geese at the waterfront. King County sometimes hangs shiny mylar strips (“scare tape”) at beaches to discourage geese flocks.
Thornton Creek Alliance has a testing program to monitor stream water quality and try to identify points where contamination is originating. This could show, for example, where a leaking sewer line is sending contaminants into Thornton Creek.
To learn more about the community-based water monitoring programs of Thornton Creek Alliance, check their website and plan to visit with them at their info table at Summerfest in Lake City on Saturday, August 1, 2026.
You can join in with Thornton Creek Alliance volunteers at upcoming work parties. One is on Saturday, August 15, at Meadowbrook Pond, from 9 AM to noon. The meeting place is 10700 36th Ave NE. The next work party will be on Saturday, August 29, from 9 AM to noon at the Kingfisher Natural Area. The meeting place is at 2003 NE 100th Street.
Sources:
King County beach status page.
King County water quality monitoring info.
Investigate the status of a Combined Sewer Overflow Outfall.
“Can Cities Grow without Harming Lakes?”, Thornton Creek Alliance meeting, April 23, 2026, with speaker Daniel Nidzgorski of the King County Department of Natural Resources & Parks.
