Claim: There is a county in Southern California.

Riverside County , California , is one of fifty-eight counties in the U.S. state of California . As of the 2010 census , the population was 2,189,641 , making it the 4th-most populous county in California and the 11th-most populous in the United States . The name was derived from the city of Riverside , which is the county seat .   Riverside County is included in the Riverside-San Bernardino-Ontario , CA Metropolitan Statistical Area , also known as the Inland Empire . The county is also included in the Los Angeles-Long Beach , CA Combined Statistical Area . There is a high concentration of sprawling tract housing communities around Riverside and along the Interstate 10 , 15 , and 215 freeways .   Roughly rectangular , Riverside County covers 7208 sqmi in Southern California , spanning from the Greater Los Angeles area to the Arizona border . Geographically , the county is mostly desert in the central and eastern portions , but has a Mediterranean climate in the western portion . Most of Joshua Tree National Park is located in the county .   The resort cities of Palm Springs , Palm Desert , Indian Wells , La Quinta , Rancho Mirage , and Desert Hot Springs are all located in the Coachella Valley region of Riverside County . Large numbers of Los Angeles area workers have moved to the county in recent years -LRB- data from the US Census Bureau for 2007 through 2011 -RRB- to take advantage of relatively affordable housing . Along with neighboring San Bernardino County , it was one of the fastest growing regions in the state prior to the recent changes in the regional economy . In addition , smaller , but significant , numbers of people have been moving into Southwest Riverside County from the San Diego-Tijuana metropolitan area . The cities of Temecula and Murrieta accounted for 20 % of the increase in population of the county between 2000 and 2007 .