Columbus is the third largest city in the American Midwest, and the fifteenth largest city in the United States of America. It is the county seat of Franklin County. The city has expanded and annexed portions of adjoining Delaware County and Fairfield County. It is named for explorer Christopher Columbus. The city was founded in 1812 at the confluence of the Scioto and Olentangy rivers. It became Ohio's state capital in 1816. The city has a diverse economy based on education, government, insurance, banking, fashion, defense, aviation, food, clothes, logistics, steel, energy, medical research, health care, hospitality, retail, and technology.