The westernmost tributary is the Illinois River, which rises in southern Josephine County, in the Red Buttes Wilderness. The river flows generally northwest along the west side of the Klamath Mountains, past Cave Junction and Kerby then through the Siskiyou National Forest. It joins the Rogue River from the south on the Curry–Josephine county line, approximately 15 mi (24 km) from the Pacific Ocean. The region is marked by its high elevation and is significantly influenced by marine climates.

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