Commissioned by President Thomas Jefferson after the Louisiana Purchase doubled the size of the United States, the expedition set out up the Missouri River in a keelboat and two pirogues. Over the next two and a half years they would travel roughly 8,000 miles, map vast unknown territory, catalog hundreds of new plant and animal species, and encounter dozens of Native American nations, guided crucially by the Shoshone woman Sacagawea.
Jefferson's instructions to Lewis were sweeping: find a water route to the Pacific, document the land's geography, climate, plants and animals, and open diplomatic and commercial relations with Native nations. The expedition left from Camp Dubois near St. Louis with a specially built 55-foot keelboat loaded with trade goods, weapons, and scientific instruments.
The journey nearly ended in disaster more than once, from a near-drowning in a squall to a tense standoff with the Teton Sioux, but the party lost only one man, Sergeant Charles Floyd, likely to a burst appendix. Sacagawea, who joined at Fort Mandan, proved indispensable both as an interpreter and as a living assurance to other tribes that the group's intentions were peaceful.
The expedition returned in September 1806 having failed to find the hoped-for all-water route to the Pacific, since the Rockies made that impossible, but its scientific and cartographic achievements reshaped American understanding of the West and accelerated the nation's westward expansion, with consequences that would prove devastating for the Native nations they encountered.