The commonly cited duration of the Lewis and Clark Expedition is 862 days.
The Corps of Discovery left the St. Louis area in May 1804 and returned in September 1806. President Thomas Jefferson commissioned the expedition after the Louisiana Purchase to explore the new western territory, map routes, study plants and animals, and establish diplomatic contact with Indigenous nations.
Meriwether Lewis and William Clark jointly led the expedition. The party traveled up the Missouri River, crossed the Rocky Mountains, reached the Pacific coast, and returned by a different route. Sacagawea accompanied the group with her infant son and helped with communication and travel.
The expedition was not a simple uninterrupted march. The travelers spent months building winter quarters, holding councils, recording observations, and navigating rivers. Sergeant Charles Floyd died in 1804, the only member known to have died during the journey. The expedition’s journals and maps greatly expanded American knowledge of the region.