The San Bernardino Mountains are a high and rugged mountain range in Southern California in the United States. Situated north and northeast of San Bernardino and spanning two California counties, the range tops out at 11,503 feet (3,506 m) at San Gorgonio Mountain – the tallest peak in all of Southern California. The San Bernardinos form a significant region of wilderness and are popular for hiking and skiing.
The mountains were formed about eleven million years ago by tectonic activity along the San Andreas Fault, and are still actively rising. Many local rivers originate in the range, which receives significantly more precipitation than the surrounding desert. The range's unique and varying environment allows it to maintain some of the greatest biodiversity in the state. For over 10,000 years, the San Bernardinos and their surroundings have been inhabited by indigenous peoples, who used the mountains as a summer hunting ground.
Spanish explorers first encountered the San Bernardinos in the late 18th century, naming the eponymous San Bernardino Valley at its base. European settlement of the region progressed slowly until 1860, when the mountains became the focus of the largest gold rush ever to occur in Southern California. Waves of settlers brought in by the gold rush populated the lowlands around the San Bernardinos, and began to tap the mountains' rich timber and water resources on a large scale by the late 19th century.
Recreational development of the range began in the early 20th century, when mountain resorts were built around new irrigation reservoirs. Since then, the mountains have been extensively engineered for transportation and water supply purposes. Four major state highways and the California Aqueduct traverse the mountains today; these developments have all had significant impacts on area wildlife and plant communities.
The San Bernardino Mountains, along with the nearby San Gabriel and San Jacinto ranges, is considered a sky island – a high mountain region whose plants and animals vary dramatically from those in the surrounding semi-arid lands. The San Bernardinos in particular comprise the largest forested region in Southern California, and support some 1,600 species of plants. Foothill regions are primarily composed of chaparral and evergreen oak woodland communities, with a transition to forests of deciduous oak, yellow pine, Jeffrey pine, incense cedar and several fir species at elevations above 5,000 feet (1,500 m). Deeper within the mountains, perennial streams fed by springs and lakes nourish stands of alders, willows and cottonwoods.
About 440 species of wildlife inhabit the mountains, including many endangered species such as the San Bernardino flying squirrel, California Spotted Owl, Mountain yellow-legged frog, Southern rubber boa, and Andrew's marbled butterfly. The mountains once had an abundant population of California grizzly, but hunting eliminated their populations by 1906. Black bears roam the highlands today, but they are not native to the region: they were imported from the Sierra Nevada by the California Department of Fish and Game in the 1930s, in part to attract tourists to the mountains.