 The river roughly defined the American frontier in the 19th century , particularly downstream from Kansas City , where it takes a sharp eastern turn into the heart of the state of Missouri . The major trails for the opening of the American West all have their starting points on the river , including the California , Mormon , Oregon , and Santa Fe trails . The first westward leg of the Pony Express was a ferry across the Missouri at St. Joseph , Missouri . Similarly , most emigrants arrived at the eastern terminus of the First Transcontinental Railroad via a ferry ride across the Missouri between Council Bluffs , Iowa and Omaha . The <unk> Bridge became the first bridge to cross the Missouri River in 1869 , and its location was a major reason why Kansas City became the largest city on the river upstream from its mouth at St. Louis . 
