Combined with the Mississippi River into which it flows as St. Louis, the Missouri drains much of the northern plains in the longest river system in America. It originates at the aptly named Three Forks of the Missouri, the spot in Montana where the Jefferson, the Madison, and the Gallatin rivers come together.
Marquette knew of the river as Peki-tan-oui and that was the name that appeared on early maps. Later, it was called Oumessourit. The French called it St. Philip. The lower part was known to French trappers, traders and voyageurs, who explored it as far up as the Kansas River in 1705.
It was first explored to its source by Lewis and Clark. The first steamboat to navigate the Missouri traveled from St. Louis to Old Franklin in thirteen days in 1819. Pierre Chouteau went still further in his steamboat Yellowstone in 1831.