How We Got Here

View a video about the history of the Seattle Sewer System and CSOs.

1856 – 1956

Sewer Outfall 1920

This 1920 photo shows a large outfall spilling untreated sewage and stormwater into Puget Sound.

For well over a century Seattle has been working on its sewer system to protect the health of people and the environment.

As a matter of civic pride, the city was motivated to make improvements when the Alaska-Yukon-Pacific Exposition hit town in 1909. As newspapers of the day reported, we risked looking “ridiculous in the eyes of the world” if we didn’t complete what was described as essential work.

About a decade later, the city started building tunnels under what is now Seattle Center and Discovery Park to pipe wastewater “out of town” and into Puget Sound. Similarly, sewage and stormwater were piped into Lake Washington and Lake Union.

1956 - 1970

Water Quality Warning

By the time of this photo (1958) Lake Washington was polluted from decades of untreated sewage spilling into its waters.

By the 1950s, Seattle had discontinued the practice of building combined sewers, opting instead to lay separate pipes to handle stormwater apart from wastewater, reducing CSOs.

In 1968 Seattle voters approved a $70 million measure to improve the city’s sewer system by separating stormwater from sewage in some parts of the city. Thanks to the Forward Thrust civic improvement campaign, voters approved funding for many improvements to the sewage collection and treatment system.

1970 – Present

Roadside Rain Garden

Roadside rain gardens feature special soils and plantings to allow stormwater to slowly seep into the ground, reducing contaminated runoff.

Since the passage of the Clean Water Act in 1972, Seattle has made many improvements to the drainage and wastewater systems. While the focus in the 1970s was on separating the drainage from the wastewater system, the 1980s saw the construction of many storage facilities, and in the past 10 years retrofits have been made to improve the system.

Seattle has been helping to lead the way to improved stormwater quality through a variety of innovative and award-winning programs. Find out more: