Lake Washington is a large, deep body of freshwater located east of Seattle. Despite its proximity to the Pacific Ocean and Puget Sound, the lake’s freshwater composition and a significant man-made barrier make it an uninhabitable environment for marine sharks. The persistent rumors about sharks in the lake are based on misidentification and a misunderstanding of the geographic and biological constraints on these marine animals.
Salinity, Biology, and Why Sharks Need Saltwater
The primary reason marine sharks cannot survive in Lake Washington is a biological process called osmoregulation. Sharks, skates, and rays—a group of fish known as elasmobranchs—maintain an internal body fluid concentration that is slightly higher than the surrounding seawater. They achieve this hyperosmotic state by retaining high levels of urea and trimethylamine oxide (TMAO) in their bloodstream and tissues. This strategy prevents the continuous loss of water from their bodies to the saltier ocean water, a problem marine fish must constantly combat.
When a typical marine shark enters a freshwater environment like Lake Washington, this delicate balance is immediately disrupted. The much lower solute concentration of the lake water causes a massive influx of water into the shark’s body across its gills and skin. Simultaneously, essential salts and the critical urea and TMAO begin to leach out into the surrounding water. The shark’s kidneys, which are adapted for salt retention, cannot process the volume of excess water fast enough, leading to cellular swelling, circulatory failure, and ultimately death.
The Bull Shark is the only exception globally, known for its ability to swim between salt and fresh water. This is accomplished by reducing urea production and producing a large volume of dilute urine through highly efficient kidneys. However, the Bull Shark is a species of warm, tropical waters and is not native to or found in the cold, temperate Pacific Northwest. The biological requirements of sharks make permanent or even temporary residence in Lake Washington impossible.
The Barrier: Lake Washington Ship Canal and Locks
Beyond the biological constraints, a significant man-made structure physically separates Lake Washington from the marine environment of Puget Sound. The Lake Washington Ship Canal connects the lake to the sound, but a complex of dams and locks, known as the Hiram M. Chittenden Locks (or Ballard Locks), acts as a barrier. Completed in 1917, the locks system serves to raise and lower vessels between the two water bodies, accommodating a difference of 20 to 22 feet in water level.
The locks facilitate boat traffic and prevent saltwater intrusion into the freshwater system. The structure is designed to hold back the high-salinity water of Puget Sound, ensuring the freshwater ecology of Lake Washington and Lake Union is maintained. Large marine animals, such as sharks, cannot pass the locks through the operational chambers, which are only opened to transit vessels one at a time.
A separate fish ladder is integrated into the locks to allow anadromous species, primarily salmon, to migrate upstream into the freshwater for spawning. This ladder is designed for the swimming and leaping ability of salmon and is not a viable pathway for large, non-leaping sharks. The combination of the physical structure and the controlled water flow ensures that Puget Sound’s saltwater inhabitants cannot breach the barrier into the lake.
Large Fish Species Mistaken for Sharks
The rumors of sharks in Lake Washington often stem from the occasional sighting of genuinely large fish that are native or established in the lake. The most frequent source of misidentification is the White Sturgeon, a prehistoric-looking species that can grow to impressive sizes. These fish are bottom-dwellers, but they occasionally surface or are found dead, leading to sensational reports.
White Sturgeon can reach lengths of over 10 feet and weigh hundreds of pounds. Their large, torpedo-shaped bodies can easily be mistaken for a shark’s silhouette. In one documented instance, an eight-foot sturgeon found in the lake was initially identified as a shark by the person who discovered it.
Another possible source of large-fish sightings is the Chinook Salmon, which enter the lake system during their spawning runs. While not as large as a sturgeon, a massive, dark-colored Chinook swimming near the surface can be mistaken for an unusual creature. These instances of misidentification are the likely origin of the enduring local folklore.