The Illinois sub-basin is a 989-square mile watershed that feeds the 56-mile long Illinois River, a tributary to the Rogue River. The headwaters drain a small portion of the Siskiyou Wilderness in Del Norte County, California, but primarily flow from the Kalmiopsis Wilderness and the slopes of the Siskiyou Crest in southern Oregon. The majority of the sub-basin is within Josephine County. The Illinois River flows into the Rogue River at Agness on the Josephine/Curry County line.
The watershed is overwhelmingly owned by the Forest Service in the upper elevations and again in the lower watershed around its confluence with the Rogue. Private ownership and BLM land is interspersed around the lightly populated Illinois Valley along Highway 199.
In 1984, Congress designated 50.4 miles of the Illinois River as Wild and Scenic. The 29-mile “wild” section flows through the Kalmiopsis Wilderness and contains 150 challenging Class III-IV rapids and one tricky Class V. Between the rapids, the river pools quietly, offering excellent spawning habitat for salmon and steelhead. Fishing for salmon, trout, and steelhead is catch-and-release only.
Tributaries to the Illinois River include Sucker, Briggs, Althouse,
Silver, Klondike, Lawson, Indigo, Josephine and Deer Creeks, in addition
to the East and West Forks of the Illinois River before they converge.
Towns in the watershed include Selma, Kerby, Cave Junction, O'Brien,
Rogue Riverkeeper regularly works in this watershed to stop harmful mining activities and gain permanent protections for the Caves Creek watershed by expanding the Oregon Caves National Monument and removing cattle from a grazing allotment around Bigelow Lakes.