Klamath_Mountains The Klamath Mountains, which include the Siskiyou, Marble, Scott, Trinity, Trinty Alps, Salmon, and northern Yolla-Bolly Mountains, are a rugged lightly populated mountain range in northwest California and southwest Oregon in the United States. The highest peaks are Mount Eddy (9025 ft / 2750 m) in Siskiyou County, California, Thompson Peak (9002Trinity County, California, and Mount Ashland (7,533Jackson County, Oregon.