Santa Ana River in the context of "Flash flood"

The Inland Empire (commonly abbreviated as the IE) is a metropolitan area and region inland of and adjacent to coastal Southern California in the Greater Los Angeles area, focusing around the cities in Riverside and San Bernardino county with Los Angeles County and Orange County to the west. The region, at its narrowest definition, includes the cities of northwestern Riverside County and southwestern San Bernardino County that are part of the contiguous urbanized area of Greater Los Angeles. It is sometimes considered to include the desert communities of the Coachella and Victor Valleys, respectively on the other sides of the San Gorgonio Pass and San Bernardino Mountains from the Santa Ana River watershed that creates the majority of the Inland Empire. A much wider definition includes the entireties of Riverside and San Bernardino counties, stretching east to the Nevada and Arizona borders and covering a larger area than West Virginia; this definition is primarily used by the US Census Bureau, which exclusively delineates metropolitan areas at the county level.

The U.S. Census Bureau–defined Riverside–San Bernardino–Ontario metropolitan area, which comprises Riverside County and San Bernardino, California, covers more than 27,000 sq mi (70,000 km) and had a population of about 4.6 million in 2020. At the end of the 19th century, the Inland Empire was a major center of agriculture, including citrus, dairy and winemaking. Agriculture declined through the 20th century and a rapidly increasing population, helped by families migrating in search of affordable housing, has led to more residential, industrial and commercial development since the 1970s.

Riverside is a city in and the county seat of Riverside County, California, United States. It is named for its location beside the Santa Ana River in Southern California. It is the most populous city in the Inland Empire and Riverside County, 12th-most populous city in California, and 61st-most populous city in the U.S. with a population of 314,998 at the 2020 census. The Riverside–San Bernardino metropolitan area at 4.74 million residents is the 12th-largest metropolitan area in the nation. Riverside is about 50 miles (80 km) east of downtown Los Angeles and is also part of the Greater Los Angeles area.

Riverside was founded in the early 1870s. It is the birthplace of the California citrus industry and home of the Mission Inn, the nation's largest Mission Revival Style building. It is also home to the Riverside National Cemetery and the Eastern Division of the Federal District Court for the Central District of California.

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.