A traditional city

The area that would become St. Louis is located on the traditional, ancestral, and unceded territory of the Illini Confederacy, a group of 12–13 Native American tribes in the upper Mississippi River valley of North America. 

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 At the time of European contact in the 17th century, they were believed to number in the tens of thousands of people, with the Grand Village of the Illinois alone having a population of about 20,000.  By the mid-18th century, only five principal tribes remained: the Cahokia, Kaskaskia, Michigamea, Peoria, and Tamaroa

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The indigenous people in the area built numerous temple and residential earthwork mounds on both sides of the Mississippi River.  Cahokia Mounds being the regional center.  The many major earthworks within St. Louis boundaries earned the the city the nickname "Mound City". These mounds were mostly demolished during the city's development.

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The gateway to the West

According to the competition's call for submissions, the new monument was to serve as “a living memorial to Jefferson's 'vision of greater opportunities for men of all races and creeds. '” Speaking of his design, Saarinen said, “The arch symbolized the gateway to the West, the national expansion, and whatnot.”