[4] It drains an area of about 773 square miles (2,000 km2) in the Central Oregon Coast Range southwest of the Willamette Valley and north of the watershed of the Umpqua River.

[5] It rises in the mountains of southwestern Lane County, about 10 miles (16 km) west of Cottage Grove.

[8] It flows generally west-northwest through the mountains, past Swisshome, entering the Pacific at Florence.

The Coos Bay branch of the Coos Bay Rail Link crosses many bridges as it follows the narrow, winding valley of the Siuslaw River to the swing bridge at Cushman.

[11] The estuary of the river is surrounded by extensive wetlands that are a significant habitat for migratory birds along the coast.