Riverside County is a county in the state of California, United States. One of 58 California counties, it covers 7,208 sq mi (18,669 km2) in the southeastern part of the state, and stretches from Orange County to the Colorado River, which forms the border with Arizona. The county derives its name from the city of Riverside, which is the county seat. The population of Riverside County was 1,545,387 in 2000, and by 2008 the U.S. Census Bureau estimated the population had risen to 2,100,516. Geographically, the county is mostly desert. Most of Joshua Tree National Park is located in the county. Riverside County lies inland of Los Angeles County, and south of San Bernardino County. Large numbers of Los Angeles workers have moved to the county in recent years to take advantage of relatively affordable housing. Alongside neighboring San Bernardino County.