Riverside is the county seat of Riverside County, California, United States. The city is named for the nearby Santa Ana River, and is the birthplace of the California citrus industry. As of 2006, Riverside had an estimated population of 305,255. blank">MONEY Magazine: Best places to live 2006: Riverside, CA snapshot Riverside is the 61st-largest city in the United States, and the largest city in California's _Inland Empire region, the 14th-largest metropolitan area in the nation and the 2nd largest CSA in the country.