The San Gabriel Valley is one of the principal valleys of Southern California, lying generally to the northeast of the city of Los Angeles. It is immediately south of the San Gabriel Mountains and is separated from the San Fernando Valley by the San Rafael Hills and the Crescenta Valley to its west, from the Los Angeles Basin by the San Rafael Hills and the Puente Hills to the south, and from the Inland Empire by the Chino Hills and San Jose Hills to the east. It derives its name from the San Gabriel River that flows southward through the center of the valley, which itself was named for the Spanish Mission San Gabriel Arcángel originally built in the Whittier Narrows in 1771. At one time predominantly agricultural, the San Gabriel Valley is today almost entirely urbanized and is an integral part of the Greater Los Angeles metropolitan area. It is one of the most ethnically diverse regions in the country. About 200 square miles (520 km2) in size, the Valley includes thirty-one cities and five unincorporated communities. In 1886, Pasadena was the first independent incorporated city still located in Los Angeles County (both Anaheim and Santa Ana are now located in Orange County).