It rises in the Coastal Range of southwestern Lane County, approximately 5 mi (8 km) west of Cottage Grove. It flows generally WNW through the mountains, past Swisshome, entering the Pacific at Florence. The head of tide is 25 miles upstream.
The valley of the river has been historically one of the productive timber regions in Oregon. The lower course of the river passes through Siuslaw National Forest.
The river has historically been a spawning ground for both Chinook and Coho salmon. Although the present Chinook population is healthy, the Coho is a mere 5% of its historic numbers. The estuary of the river is surrounded by extensive wetlands that are a significant habitat for migratory birds along the coast.
