St. Louis It is located on the west side of the Mississippi River, which forms the Illinois-Missouri state line. The Missouri River merges 15 miles north of downtown St. Louis with the Mississippi River creating the world's fourth longest river system. The City's total population is more than 302,000. St. Louis was founded in 1764 by Pierre Laclde and Auguste Chouteau, French fur traders, and named after Louis IX of France. The dynamic economy of St. Louis has strengths in the retail, manufacturing, trade, transport, and tourism industries. Nine Fortune 500 companies based in Missouri are located in the Metropolitan area. The total area of St. Louis is 65 miles.