Barker Dam Area in Joshua Tree National Park. Joshua Tree National Park is located in southeastern California. Declared a U.S. National Park in 1994 when the US Congress passed the California Desert Protection Act (Public Law 103-433), it had previously been a U.S. National Monument since 1936. It covers a land area of 789,745 acres (319,598 ha). A large part of the park is designated to wilderness area; some 429,690 acres (173,890 ha). Straddling the San Bernardino County/Riverside County border, the park includes parts of two deserts, each an ecosystem whose characteristics are determined primarily by elevation. Below 3,000 feet (910 m), the Colorado Desert encompasses the eastern part of the park and features natural gardens of creosote bush, ocotillo, and cholla cactus. The Little San Bernardino Mountains run through the southwest edge of the park.