The Columbia River Estuary IBA runs from the Columbia River Plume (consistent with BirdLife International's Marine IBA designation) to approximately river mile 60 at Crims Island (near Mayger, Oregon, and across from Stella, Washington), consistent with Western Hemisphere Shorebird Reserve Network designation.
The Columbia River Plume is a transition zone where freshwater from the Columbia River mixes with saltwater from the ocean. The area occupied by the plume is highly dynamic dependent upon the season, hydropower spill, wind direction, and ocean currents. In general, the summer Plume pushes southward and occurs regularly offshore between Cape Meares, Oregon and Grays Harbor, Washington, while in winter it is regularly found northward and inshore. From Guy, Zamon and Hunter nomination, Jan 2008.
Regular seabird and marine mammal strip-transect counts have been conducted by NOAA Fisheries and OSU each June from 2003-2007. These surveys consistently record higher numbers of seabirds using the area occupied by the Columbia River Plume where northern anchovy, a major diet item, spawn. Seabird counts are routinely high when transects cross strong salinity gradients. Strong gradients are most often encountered at Willapa Bay, Columbia River, and Cape Meares. Sooty Shearwaters and Common Murres are the numerically dominant seabird species in all areas surveyed. California Gulls, Brown Pelicans, Caspian Terns, Heerman's Gulls, DC Cormorants, Pelagic Cormorants, and Brandt's Cormorants also use the areas in spring, summer and fall. Marbled Murrelet and Rhinoceros Auklet are occasionally observed in the plume.
Bimonthly surveys at the mouth of the Columbia River are performed at the North Head Lighthouse in Washington by NOAA Fisheries and OSU. These land-based surveys consistently document large aggregations of seabirds that apparently come to feed at the Col River Plume throughout the year. It is not uncommon to observe 1,000s to 10,000s of Sooty Shearwaters between April and September in the vicinity of the river mouth and plume. Common Murres, Double-crested Cormorants, Pacific Loons, Red-throated Loons, and Western Grebes also use the area and have been recorded in the 100s to 1,000s as the plume front passes the survey area.