U.S. negotiators Robert R. Livingston and James Monroe and French minister François Barbé-Marbois sign the Louisiana Purchase treaty, agreeing to transfer Louisiana from France to the United States for $15 million—dramatically expanding U.S. territory west of the Mississippi River.
The U.S. Senate votes to approve the treaty for ratification, clearing the major constitutional and political hurdle for the acquisition of Louisiana.
After Senate consent and exchange of ratifications, President Thomas Jefferson issues a proclamation making the Louisiana Purchase treaty and accompanying conventions public and directing U.S. officials and residents to observe them.
In New Orleans, Louisiana is formally transferred to U.S. control in a public ceremony involving French and U.S. officials, marking the practical change of sovereignty in the lower territory.
At St. Louis, authority over Upper Louisiana passes through a brief French interlude to the United States during the "Three Flags" ceremonies, completing U.S. assumption of control in the upper portion of the purchase.
The expedition’s “point of departure” occurs as William Clark and the Corps leave Camp Dubois (near the Mississippi–Missouri confluence) and begin traveling up the Missouri River to explore the Louisiana Purchase and seek a route to the Pacific.
Sergeant Charles Floyd dies while the Corps travels up the Missouri River—the expedition’s only fatality—highlighting the medical risks of long-distance exploration in the early republic.
At Fort Mandan, the Corps adds interpreter Toussaint Charbonneau and Sacagawea, whose language skills and connections to Shoshone communities would be crucial for obtaining horses and negotiating passage westward.
The Corps completes Fort Mandan near the Mandan and Hidatsa villages and settles in for the winter of 1804–1805, enabling rest, repairs, diplomacy, and planning for the Rocky Mountain crossing.
Meriwether Lewis and a small advance party cross the Continental Divide at Lemhi Pass, encountering the reality of multiple western mountain ranges and advancing efforts to contact Shoshone groups for horses.
William Clark writes “Ocian in view! O! the joy.” upon reaching the Columbia River estuary—close to, but not yet at, the open Pacific Ocean—signaling the expedition’s approach to its westward goal.
Near the mouth of the Columbia, Clark records the party’s collective input on where to winter, with York and Sacagawea included—an unusual moment of broad participation in expedition decision-making.
After selecting a wintering site on the south side of the Columbia River, the Corps starts construction of Fort Clatsop, a temporary post intended to shelter the party through the wet coastal winter while they organize notes, maps, and specimens.
The Corps leaves Fort Clatsop at about 1 p.m. to begin the long return to the United States, concluding the expedition’s winter on the Pacific coast and shifting focus to safe passage back across the continent.
The Corps of Discovery arrives back in St. Louis, completing a transcontinental journey that produced extensive geographic, scientific, and diplomatic information and strengthened U.S. claims and knowledge across the Louisiana Purchase and beyond.
Louisiana Purchase and the Lewis and Clark Expedition (1803–1806)
