The Alsea River flows 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. Its drainage basin extends into Lane County, along the headwaters of the South Fork Alsea River.
The Alsea River supports runs of chinook and coho salmon, as well as steelhead and coastal cutthroat trout. The Alsea River Fish Hatchery is along one of its tributaries, the North Fork Alsea River.
Course
The Alsea River begins in the Central Oregon Coast Range near the unincorporated community of Alsea, and flows generally west about to the Pacific Ocean at Waldport, on the west coast of the United States. Formed by the confluence of the South Fork Alsea River and the North Fork Alsea River, the stream meanders through the Siuslaw National Forest for all but the last of its course. Oregon Route 34 follows the river closely, crossing it once about from the mouth. Along this stretch, which is about long, the river passes under Hayden Covered Bridge; receives Schoolhouse Creek from the right, Birch Creek from the left, and Narrows Creek and Maltby Creek from the right, passes Campbell Boat Landing on the right and Salmonberry Boat Landing on the left, goes under Salmonberry Road, receives Salmonberry Creek from the left, passes Missouri Bend Recreation Site and Boat Ramp on the left, and receives Benner Creek from the left and Digger Creek from the right. on the right about from the mouth. About later, the river begins another U-shaped loop of about around the south end of Stoney Mountain, which rises to above sea level.
Flowing around Stoney Mountain, the Alsea River receives Five Rivers and Lake Creek from the left, passes Blackberry Campground on the left, receives Grass Creek and Brush Creek from the left and Schoolhouse Creek from the right, passes Mike Bauer Picnic Grounds on the right and, shortly thereafter, a USGS gauging station on the right from the mouth. About further downstream, the river passes Slide Campground on the right before entering Hellion Rapids at coordinates . Downstream of the rapids, the river receives Scott Creek, Slide Creek, and Hatchery Creek, all from the right, and Line Creek from the left before reaching the unincorporated community of Tidewater, about from the mouth.