The flag of St. Louis, Missouri was designed by Yale University professor Theodore Sizer. The flag is often called the "Three Rivers" flag, symbolizing the confluence of the Mississippi, Missouri, and the Meramec rivers near St. Louis. The intersection of the blue lines is covered by a fleur-de-lis, which symbolizes St. Louis's French heritage. Although the flag is strictly the municipal flag of St. Louis, it is sometimes flown over government buildings in St. Louis County.
Revised Code of the City of St. Louis, (Section 1.20.010)
"The design submitted by Professor Emeritus Theodore Sizer, Pursuivant of Arms at Yale university, and now on file in the office of the City register is approved, adopted and designated as the official flag of the City. The flag with a solid red background has two broad heraldic wavy bars, colored blue and white, extending from the left top and bottom corners toward left center where they join and continue as one to the center right edge. This symbolizes the confluence of the Missouri and Mississippi rivers. Over the point of confluence a round golden disk upon which is the fleur-de-lis of France (blue) calling attention to the French background of the early city and more particularly to St. Louis of France for whom the City is named. The golden disk represents the City and/or the Louisiana Purchase. (Heraldically, the disk is a "bezant" or Byzantine coin signifying, money or simply purchase.)
The flag's colors recall those of Spain (red and yellow or gold), Bourbon France (white and gold), Napoleonic and Republican France (blue, white and red), and the United States of America (red, white, and blue)."
taken from Wikipedia