Port Susan Bay holds some of the finest estuarine habitat in Puget Sound. Its marshes, vast mudflats and tidally influenced channels support hundreds of thousands of birds, several species of salmon, smelt, English sole and clams. Western sandpipers, dunlins and dowitchers swoop over the mudflats. Wrangel Island snow geese gather by the thousands in tidal marshes and on nearby farm fields. And hundreds of raptors, from peregrine falcons to short-eared owls, add to the drama.
The Stillaguamish River spills into the bay, mixing freshwater and saltwater to create extensive estuarine marshes that produce a vast quantity of decaying organic matter, which feeds the abundant invertebrate life in the tide flat sediments. These tiny creatures, in turn, feed the shorebirds and waterfowl that make Port Susan Bay and adjacent Skagit Bay important stops for migratory birds traveling along the Pacific Flyway.