When the last glaciers receded from Washington some 15,000 years ago, they left behind a deep inland sea, Puget Sound, which today is Washington's most unique and prominent natural feature. Wedged between the Olympic and Cascade mountain ranges, the Puget Sound basin is protected from the brunt of Pacific Ocean storms, giving it an ideal temperate climate. The Puget Sound region, with more than 1,000 miles of saltwater shoreline and many large islands, provides some of the most important wildlife habitat and best wildlife viewing opportunities in the western United States.

The waters and shoreline of Puget Sound make up a biologically-diverse ecosystem. The Sound is home to at least 211 species of marine fish, and a variety of marine birds, waterfowl, and marine mammals, including orca whales, California sea lions, and a large population of harbor seals. Rivers flowing into Puget Sound provide important migration corridors and spawning habitat for Pacific salmon. River estuaries are critical resting and feeding areas for ducks, geese, swans, and shorebirds migrating north and south along the Pacific Flyway.