Six Rivers National Forest is a U.S. National Forest located in the northwestern corner of California. It was established on June 3, 1947 by U.S. President Harry S. Truman from portions of Klamath, Siskiyou and Trinity National Forests. Its over one million acres (4,000 km²) of land contain a variety of ecosystems and 137,000 acres (550 km²) of old growth forest. It lies in parts of four counties; in descending order of forestland area they are Del Norte, Humboldt, Trinity, and Siskiyou counties.