Puget Sound is a vast, deep inlet of the Pacific Ocean, stretching through the American Pacific Northwest from the Strait of Juan de Fuca down to Olympia, Washington. This waterway, part of the larger Salish Sea, was formed by ancient glaciers that carved out deep troughs. The question of whether the sound contains salt water or fresh water does not have a simple binary answer. It is defined by continuous mixing, creating an environment that is neither purely oceanic nor purely riverine.
Puget Sound is an Estuary
Puget Sound is classified as an estuarine system, a semi-enclosed coastal body where sea water is significantly diluted by freshwater runoff. This means the sound is an active zone of water blending, resulting in a brackish environment. The water is saline, but substantially less salty than the open Pacific Ocean.
Ocean water typically maintains a salinity of approximately 34 parts per thousand (ppt). In contrast, the average salinity of Puget Sound is lower, measuring around 28.5 ppt, roughly 83% of the Pacific’s concentration. This average is not uniform; it varies widely based on location and depth. The sound’s physical structure is that of a fjord, a deep U-shaped basin, which contributes to how the fresh and salt waters interact.
Hydrological Drivers of Salinity
The fluctuating salinity levels are maintained by two hydrological forces: river inflow and tidal exchange with the Pacific Ocean. The sound’s watershed receives freshwater from 19 major river basins, carrying snowmelt and rainfall from the Cascade and Olympic Mountains. The mean annual river discharge into the sound is substantial, averaging 41,000 cubic feet per second.
The Skagit River is the largest source of freshwater, contributing roughly 30% of the total runoff. This influx of less-dense freshwater pushes the overall salinity downward, particularly near the surface and river deltas. Competing with this runoff is ocean water exchanged with the Pacific via the Strait of Juan de Fuca and Admiralty Inlet.
This tidal exchange is the primary mechanism for flushing the sound, pulling dense, salty Pacific water into the depths and pushing the diluted surface water back out. The flow exchanged at Admiralty Inlet is estimated to be 20 to 30 times greater than the total river flow. This powerful exchange rapidly reduces the water’s residence time to 90 to 180 days, ensuring the water is regularly renewed and remains predominantly saline.
Water Density and Stratification
The difference in density between freshwater and ocean water creates the sound’s layered structure, known as stratification. Salinity, more than temperature, is the main control on water density in this estuarine environment. Less-dense, fresher water tends to float on top of the colder, denser, more saline ocean water.
This layering creates a distinct two-layer system, separating the surface water from the deep water. The boundary between these layers is the halocline, a zone where salinity increases rapidly with depth, often located between 10 and 20 meters below the surface. This stratification resists vertical mixing driven by winds and tides, effectively isolating the dense bottom layer.
This limited vertical mixing has ecological effects by controlling the movement of nutrients and oxygen within the water column. The isolation of the deep layer means that the bottom water is only slowly replaced by incoming dense ocean water.
Marine Life Adapted to Variable Salt Levels
The brackish environment of Puget Sound necessitates biological adaptation for the organisms that live there. Marine life must be able to tolerate wide fluctuations in salinity, particularly near river mouths and surface waters. Organisms that can survive across this wide range of salinity are known as euryhaline species.
The sound is a nursery and migratory pathway for numerous species. Pacific salmon are diadromous fish, meaning they migrate between fresh and salt water during their life cycle. Juvenile salmon must undergo osmoregulation to transition their bodies from freshwater rivers to the saline sound.
Other inhabitants, including shellfish and crabs, also exhibit adaptations to cope with changing salt concentrations. Their ability to survive in a habitat constantly shifting between marine and riverine influences highlights the biological richness of this estuarine system.