An estuary is the area where the fresh water of a river meets and mingles with the salt water of the ocean. Near the mouth of the Columbia the estuary waters have nearly the same salinity as the ocean, while upstream the salt water is extremely dilute. The Pacific Ocean’s saline influence over the Columbia River Estuary extends to a point just east of Puget Island. Within the river, the ocean’s tidal action can be measured up to the base of Bonneville Dam, which is approximately 120 river miles upstream from the mouth of the Columbia.

These dynamic conditions result in some of the most biologically productive ecosystems in the world because there is a large and concentrated supply of the nutrients from both the ocean and the river. The estuary provides important habitat for juvenile salmon as well as microscopic organisms, shellfish, mammals, and birds, including the great American bald eagle.