This magnificent arch rises on the banks of the Mississippi River in St. Louis, Missouri. The nation’s tallest monument is the centerpiece of the Jefferson National Expansion Memorial, honoring Thomas Jefferson’s vision of westward expansion. This magnificent arch rises on the banks of the Mississippi River in St. Louis, Missouri. The nation’s tallest monument is the centerpiece of the Jefferson National Expansion Memorial, honoring Thomas Jefferson’s vision of westward expansion. Despite its controversial beginning, this amazing structure is now a national treasure and symbol of the nation’s reach from the Atlantic to the Pacific.