The Illinois River is a tributary, about 56 miles (90 km) long, of the Rogue River in the U.S. state of Oregon. It drains part of the Klamath Mountains in northern California and southwestern Oregon. The river's main stem begins at the confluence of its east and west forks near Cave Junction in southern Josephine County. Its drainage basin includes Sucker Creek, which rises in the Red Buttes Wilderness, near on the California state line. The main stem flows generally northwest in a winding course past Kerby and through the Siskiyou National Forest and Kalmiopsis Wilderness. It joins the Rogue River from the south at Agness on the Curry–Josephine county line, 27 miles (43 km) from the Pacific Ocean.