St. Louis is a city on the eastern border of Missouri, United States. With a population of 319,294, it was the 58th-largest U.S. city at the 2010 U.S. Census. The city of St. Louis was founded in 1764 by Pierre Laclède and Auguste Chouteau, and after the Louisiana Purchase, it became a major port on the Mississippi River. Its population expanded after the American Civil War, and it became the fourth-largest city in the United States in the late 19th century. The city is commonly identified with the Gateway Arch, part of the Jefferson National Expansion Memorial in downtown St. Louis.