Los Angeles County borders 70 miles of coast on the Pacific Ocean and encompasses mountain ranges, valleys, forests, islands, lakes, rivers, and desert. The Los Angeles River, Rio Hondo, the San Gabriel River and the Santa Clara River flow in Los Angeles County, while the primary mountain ranges are the Santa Monica Mountains and the San Gabriel Mountains. The western extent of the Mojave Desert begins in the Antelope Valley, in the northeastern part of the county.

Most of the population of Los Angeles County is located in the south and southwest, with major population centers in the Los Angeles Basin, San Fernando Valley and San Gabriel Valley. Other population centers are found in the Santa Clarita Valley, Pomona Valley, Crescenta Valley and Antelope Valley.

Los Angeles County includes San Clemente Island and Santa Catalina Island, which are part of the Channel Islands archipelago off the Pacific Coast. Wikipedia