Lake Washington is a large, freshwater lake located in King County, Washington, bordering Seattle and its eastern suburbs. It is the second-largest natural lake in the state, covering 33.8 square miles and reaching a maximum depth of 214 feet. Its freshwater composition is maintained by a constant influx of surface runoff and managed outflow, which keeps its waters separate from the nearby saltwater Puget Sound.
Classification as a Freshwater Body
Lake Washington is classified as freshwater due to its low salinity, which is less than 0.5 parts per thousand. This composition results from its primary water sources: precipitation and surface runoff from the surrounding watershed. The lake receives the majority of its inflow from two main river systems: the Cedar River (57% of annual volume) and the Sammamish River (27% of total load).
The Cedar River enters the lake near Renton, and the Sammamish River flows in from the north after draining Lake Sammamish. These substantial river inflows continuously flush the lake with fresh water, overriding any minimal saline influence. Historically, the lake suffered from pollution, moving from an oligotrophic state in the 1930s to a eutrophic state in the 1950s due to sewage discharge.
A massive sewage diversion project in the 1960s successfully removed nutrients like phosphorus, allowing the lake to recover its water quality. The lake is now classified as mesotrophic, reflecting an intermediate level of productivity and water clarity.
The System Preventing Saltwater Intrusion
Despite its proximity to the saltwater Puget Sound, Lake Washington’s freshwater status is artificially protected by the Lake Washington Ship Canal. This eight-mile channel connects the lake to the Puget Sound via Lake Union and Salmon Bay. The man-made connection required a mechanism to manage the significant difference in water level between the lake and the tidal sound.
The Hiram M. Chittenden Locks, commonly known as the Ballard Locks, provide this separation. The locks maintain Lake Washington’s water level 16 to 22 feet above mean sea level, substantially higher than the tidal Puget Sound. This system functions as a barrier, preventing higher-density tidal saltwater from moving upstream into the freshwater basin.
The lock complex features two parallel chambers for raising or lowering vessels traveling between the two bodies of water. A unique drainage system is integrated to further reduce saltwater intrusion. This system collects any heavier saltwater that seeps in and transports it back into the Puget Sound via pipes, protecting the inland freshwater ecology. The entire system ensures a one-way flow of fresh water out of the lake and into the Sound, which preserves the lake’s non-saline environment.
Aquatic Life in Lake Washington
The freshwater environment of Lake Washington supports a diverse array of commercially and recreationally important species. Numerous species of salmon, such as Coho, Chinook, and Sockeye, utilize the lake and its tributaries for spawning runs, migrating through the locks. Landlocked Sockeye, known as Kokanee, also inhabit the lake, spending their entire lives in the freshwater.
The lake is also home to a naturally sustaining population of Coastal Cutthroat Trout, which are identifiable by the distinct red slashes under their lower jaws. Popular non-native game fish thrive in the habitat, including Smallmouth and Largemouth Bass. Other common freshwater inhabitants include Yellow Perch, Black Crappie, and various carp and sucker species. The ecosystem also includes invertebrates like filter-feeding Daphnia and various insect larvae.