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. During its first or so, the river receives Cathcart Creek, Roberts Creek, and Mill Creek, all from the, and passes Mill Creek Boat Landing on the right. Oregon Route 34 parallels the river on the north; that is, on the right facing downstream. Here the river begins a long, roughly U-shaped loop around the south end of Digger Mountain, which rises to above sea level at coordinates. Along this stretch, which is about long, the river passes under Hayden Covered Bridge; receives Schoolhouse Creek from the right, Birch Creek from the, 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. Slightly west of the mountain, about from the mouth, the river leaves Benton County, and enters Lincoln County. Passing Little Digger Mountain on the right, the river receives Fall Creek from the right, Cow Creek and Minotti Creek from the left, and Wolf Creek, Phillips Creek, Cedar Creek, and Butter Creek from the right as it passes Trenholm Saddle, a mountain gap at coordinates 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. Between Tidewater and Alsea Bay, the river receives Mill Creek and Squaw Creek from the right, Canal Creek from the left, Risley Creek from the right, passes under Oregon Route 34, which then parallels the river on the south or left, and receives Arnold Creek from the left at about from the mouth. About later, it leaves the Siuslaw National Forest. Shortly thereafter, it receives Southworth Creek and Constantine Creek from the left and Drift Creek from the right. It receives Eckman Creek from the left, passes W. B. Nelson State Recreation Site on the left about from the mouth and enters Alsea Bay. As part of the bay, it receives Lint Creek from the left, passes Waldport on the left, flows under U.S. Route 101, and enters the Pacific Ocean at the mouth of the bay.
