Puget Sound is a complex marine environment in Washington State, forming a major inlet of the Pacific Ocean and part of the larger Salish Sea. Extending approximately 100 miles from the north down to Olympia, this waterway is a system of interconnected channels, bays, and deep basins. The Sound functions as a partially mixed estuarine system, where freshwater runoff meets and mixes with salty ocean water. The depths of the Sound are the result of powerful geological forces that shaped the region thousands of years ago.
The Sound’s Maximum Depth and Average Measurements
The deep, fjord-like structure of the Puget Sound gives it an impressive maximum depth. The average depth is approximately 450 feet (140 meters), which is deep compared to most estuaries globally. The deepest point is located in the Central Basin, specifically off Jefferson Point between Indianola and Kingston, registering a maximum depth of 930 feet (283 meters). The main basin, running from the southern tip of Whidbey Island toward Tacoma, maintains a depth of approximately 600 feet (180 meters). The presence of several distinct, deep basins separated by shallower ridges, known as sills, contributes to the variance in depth measurements.
Geological Formation by Glacial Carving
The profound depths of Puget Sound are a consequence of continental ice sheets during the Pleistocene Epoch. The most recent event was the Vashon Glaciation, a massive lobe of the Cordilleran Ice Sheet that advanced into the region 17,000 to 15,000 years ago. This ice sheet, known as the Puget Lobe, was thousands of feet thick, estimated up to 3,000 feet deep near present-day Seattle. The weight and movement of this massive glacier scoured the underlying bedrock of the Puget Lowland, carving out deep, linear troughs that run north-to-south and define the Sound as a classic fjord system. As the climate warmed and the ice retreated, glacial meltwater helped to further erode the topography, forming proglacial freshwater bodies like Glacial Lake Russell before the ocean flooded the system.
Deep Water Circulation and Tidal Exchange
The glacier-carved topography, characterized by deep basins separated by shallower sills, dictates the Sound’s hydrodynamic properties. Sills, particularly the one at Admiralty Inlet, restrict water exchange with the Strait of Juan de Fuca and the Pacific Ocean. This restriction creates a two-layered estuarine circulation pattern: fresher water flows out near the surface, while colder, denser, saline ocean water flows landward along the bottom. The shallow sills limit the mixing of these two layers, causing stratification as deep basins retain cold, dense water with a long residence time. This slow renewal rate is the primary mechanism for bringing oxygen to the lower depths and circulating nutrients; for instance, the deep water in Southern Hood Canal can have an annual maximum residence time of up to 99 days.