Mission San Gabriel Arcángel in the context of "Mission Vieja"

The San Gabriel Valley (Spanish: Valle de San Gabriel), sometimes referred to by its initials as SGV, is one of the principal valleys of Southern California, with the city of Los Angeles directly bordering it to the west and occupying the vast majority of the southeastern part of Los Angeles County. Surrounding landforms and other features include:

• the San Gabriel Mountains to the north;
• the San Rafael Hills to the west, with the Los Angeles Basin beyond;
• the Crescenta Valley to the northwest;
• the Puente Hills to the south, with the coastal plain of Orange County beyond;
• the Chino Hills and San Jose Hills to the east, with the Pomona Valley and Inland Empire beyond; and
• the city limits of Los Angeles bordering its western edge.

The San Gabriel Valley 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.

The San Gabriel River is a mostly-urban waterway flowing 58 miles (93 km) southward through Los Angeles and Orange Counties, California, in the United States. It is the central of three major rivers draining the Greater Los Angeles area, the others being the Los Angeles River and Santa Ana River. The river's watershed stretches from the rugged San Gabriel Mountains to the heavily developed San Gabriel Valley and a significant part of the Los Angeles coastal plain, emptying into the Pacific Ocean between the cities of Long Beach and Seal Beach.

The San Gabriel once ran across a vast alluvial flood plain, its channels shifting with winter floods and forming extensive wetlands along its perennial course, a relatively scarce source of fresh water in this arid region. The Tongva and their ancestors inhabited the San Gabriel River basin for thousands of years at villages like Puvunga, relying on the abundant fish and game in riparian habitats. The river is named for the nearby Mission San Gabriel Arcángel, established in 1771 during the Spanish colonization of California. Its water was heavily used for irrigation and ranching by Spanish, Mexican, and American settlers before urbanization began in the early 1900s, eventually transforming much of the watershed into industrial and suburban areas of greater Los Angeles.