When the United States completed the Louisiana Purchase in 1803, President Thomas Jefferson chose his personal secretary, Meriwether Lewis, to lead an expedition into the new territory. Lewis’ mission was to explore the natural resources and peoples of the new land and to:
… explore the Missouri River and such principal stream of it, as, by its course and communication with the waters of the Pacific Ocean, may offer the most direct and practical water communication across the continent, for the purposes of commerce.
Lewis selected William Clark, a comrade from the army, to lead the expedition with him.
Upon entering eastern Montana, the Corps encountered several river confluences forcing them to make critical decisions about the correct route to the headwaters of the Missouri. The captains hoped to encounter native tribes along the upper Missouri to assist them on their journey. To their dismay they found no people along the river bottoms, just an abundance of evidence that Indians were in the area at other times of the year.