The Alsea River flows 48.5 miles (78.1 km) from Alsea, an unincorporated community in the coastal mountains of the U.S. state of Oregon, to the Pacific Ocean near the city of Waldport. It begins at the confluence of the North Fork Alsea River and the South Fork Alsea River and ends in Alsea Bay, a wide estuary at Waldport. The river flows generally west-northwest in a winding course through the mountains of southern Benton and Lincoln counties, passing near the unincorporated community of Tidewater and through the Siuslaw National Forest.