The county has 3,121,251 people, spread across just 948 square miles (2,455 square kilometers). It is the third most populous county in California. The west-flowing Santa Ana River divides the county into north and south parts. On the north, the San Gabriel River forms its border with Los Angeles County. Some other big cities include Anaheim, Orange, Huntington Beach, Fullerton and Irvine. The county is mostly residential.