The San Bernardino Mountains are a large mountain range located in San Bernardino County with a small southern portion located in Riverside County. The Southern California mountain range, which borders on the San Gabriel Mountains is approximately 60 miles in length and averages 41 miles wide. The highest peak is San Gorgonio Mountain that rises to 11,503 feet. Most of the mountain range lies within the San Bernardino National Forest.

Cities that lie at the base of the San Bernardino Mountains include San Bernardino, Yucca Valley, and Hesperia. Communities that are located within the mountain range include Big Bear City, Big Bear Lake, Lake Arrowhead, Running Springs, and Crestline.

The Santa Ana River has its headwaters in the San Bernardino Mountains. The river runs westward through Orange County into the Pacific Ocean. The Whitewater River originates in the San Bernardino Mountains and flows to the southeast through the Coachella Valley into the Salton Sea. while the Mojave River flows to the north into the Mojave Desert where it fills numerous desert aquifers.