[4] 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 begins in the Central Oregon Coast Range near the unincorporated community of Alsea, and flows generally west about 48.5 miles (78.1 km) to the Pacific Ocean at Waldport, on the west coast of the United States.

Oregon Route 34 follows the river closely, crossing it once about 8 miles (13 km) from the mouth.