The Salmon River is a 19.6-mile-long (31.5 km) tributary to the Klamath River in western Siskiyou County, California. The river has its origins in the high mountains of the Trinity Alps, Russian Mountains, and Marble Mountains (all sub-ranges of the larger Klamath Mountains). The Salmon River comprises two forks, the North Fork and the South Fork, which join at the hamlet of Forks of Salmon, California to form the mainstem Salmon River. A large tributary stream, Wooley Creek, joins the mainstem Salmon River about 4 miles (6 km) from its mouth at Somes Bar, and is nearly as large as the North Fork. The lower portion of the Salmon River's southwestern divide defines the boundary of Siskiyou County and Humboldt County.