Many cities lie at the base of the San Bernardino Mountains. These include San Bernardino, Redlands and Yucaipa in the south; Yucca Valley to the east; and Hesperia to the northwest. In addition, there are several mid-sized to large towns in the mountains themselves, including Big Bear Lake, Big Bear City, Crestline, Lake Arrowhead and Running Springs. Cities within the San Bernardino Mountains total a population of about 44,000, with this number sometimes increasing tenfold during peak tourist season. Several regional streams and rivers also have their headwaters in the mountains. The principal drainage is provided by the Santa Ana River, which runs westwards into the Pacific Ocean in Orange County. Other streams flowing off the mountains include the Whitewater River, flowing southeast through the Coachella Valley into the Salton Sea, and the Mojave River, which drains northwards into the Mojave Desert.