Memphis is a city in the southwest corner of the U.S. state of Tennessee, and the county seat of Shelby County. The city is located on the 4th Chickasaw Bluff, south of the confluence of the Wolf and Mississippi rivers. Memphis has an estimated population of 670,100 making it the largest city in the state of Tennessee, the third largest in the Southeastern United States, and the 19th largest in the United States. The city's central location has led to much of its business development. Located on the Mississippi River and intersected by several freight railroads and two Interstate highways, Memphis is ideally located for commerce in the transportation and shipping industry. River barges are unloaded onto trucks and trains. The city is home to Memphis International Airport, the world's busiest cargo airport, which serves as the primary hub for FedEx Express shipping and became a secondary hub for Delta Air Lines after it merged with Northwest Airlines in 2008. Memphis is the home of three Fortune 500 companies. In 2000 Inc. magazine rated Memphis in the top eight of the 50 best major U.S. metro areas for starting and growing a business.