Eld Inlet is a long, shallow bay that lies at the south end of Puget Sound, eight kilometers northwest of Olympia. It contains extensive tidelands and marine waters within a lowland Douglas-fir forest. Mud Bay, in the south inlet, consists of extensive mudflats at low tide. There is little salt marsh, but estuarine habitat occurs at the mouths of two creeks that feed the inlet. About 80 percent of the inlet is less than 10 meters deep. In the north half of the inlet, tidelands are privately owned and used in commercial shellfish operations. Mud Bay tidelands are owned by the state.
Eld Inlet/Mud Bay IBA supports significant concentrations of wintering waterfowl, with dominant species being scoters, Buffleheads, and Ruddy Ducks. This IBA is among the most important inland sites in the Pacific Northwest for Greater Yellowlegs. During the breeding season, substantial numbers of Band-tailed Pigeons rely on the site's mineral springs for nutrients.
The greatest threat to the inlet is the impact of increasing urban and residential development in the surrounding watershed, including declining water quality from erosion; and runoff caused by grading, building, and landscaping. Runoff from pesticides and fertilizers may cause accumulations of these materials in the tideflats and may detrimentally impact the invertebrate populations that provide food for wintering birds.