The Salmon River is a 19.6-mile-long 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. 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 19.…The Salmon River is a 19.6-mile-long 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. 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 19.6-mile long mainstem Salmon River. A large tributary stream, Wooley Creek, joins the mainstem Salmon River about 4 miles 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.