valley in southern California, U.S. It lies northwest of downtown Los Angeles, bounded by the San Gabriel (San Gabriel Mountains) (north and northeast), Santa Susana (north), and Santa Monica (south) mountains and the Simi Hills (west). The valley, originally an agricultural area, occupies 260 square miles (670 square km) and is the location of several Los Angeles suburban residential communities including Encino, North Hollywood, San Fernando, Studio City, Tarzana, Van Nuys, and Woodland Hills. Primary drainage is provided by the Los Angeles River. Several water-storage reservoirs and a large flood-control dam and reservoir are located there.