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.

Modoc County is a county in the far northeast corner of the U.S. state of California. Its population is 8,700 as of the 2020 census, down from 9,686 from the 2010 census. This makes it California's third-least populous county. The county seat and only incorporated city is Alturas. Previous county seats include Lake City and Centerville. The county borders Nevada and Oregon.