Lost River begins and ends in a closed basin in northern California and southern Oregon in the United States.
The river, 60 miles (97 km) long, flows in an arc from Clear Lake Reservoir in Modoc County, California, through Klamath County, Oregon, to Tule Lake in Siskiyou County, California. About 46 mi (74 km) of Lost River are in Oregon, and 14 miles (23 km) are in California.
