What was the purpose of the Lewis and Clark Expedition?

The purpose of the Lewis and Clark Expedition was to explore the newly acquired Louisiana Territory and beyond, find a practical river route to the Pacific, gather scientific information about plants, animals, and geography, establish American claims and diplomatic relationships with Native nations, and assess prospects for trade and future settlement. President Thomas Jefferson laid out detailed instructions in a letter to Meriwether Lewis dated June 20, 1803, before the Senate had even ratified the Louisiana Purchase. Jefferson requested Congress to fund the expedition in a confidential message on January 18, 1803, and Congress quietly appropriated 2,500 dollars for the trip, although total costs eventually reached about 38,000 dollars.

The first stated purpose was geographic. Jefferson wanted to find the most direct and practicable water communication across this continent, for the purposes of commerce. Many Americans hoped the Missouri River and a short land portage would connect to the Columbia River system, providing a Northwest Passage by inland waterway. Lewis and Clark's journey demonstrated that no such easy route existed. The Bitterroot Mountains required a brutal eleven day land crossing in September 1805, and the Continental Divide and intervening ranges proved formidable.

The second purpose was scientific. Jefferson, an Enlightenment naturalist, instructed Lewis to record the soil and face of the country, its mineral productions, animals, plants, climate, prevailing winds, and to keep careful daily journals. The expedition recorded 178 new plants and 122 new animals, including the prairie dog, grizzly bear, pronghorn antelope, and bighorn sheep. They drew detailed maps that displaced earlier European fantasy geography.

The third purpose was diplomatic. Jefferson wanted Lewis to establish American sovereignty over Native peoples whose territories now technically belonged to the United States, to inform them of the change in flag, to invite chiefs to visit Washington, and to encourage trade through American merchants rather than British or Spanish traders. The Corps held formal councils with the Oto, Missouri, Yankton Sioux, Mandan, Hidatsa, Shoshone, Salish, Nez Perce, Chinook, Clatsop, and others.

The fourth purpose was commercial reconnaissance, especially for the fur trade. Trappers and merchants would soon follow the expedition's routes to harvest beaver, otter, and other pelts. The fifth purpose was strategic. By traveling overland to the Pacific, the United States laid claim to the Oregon Country, which Britain, Spain, Russia, and the United States all claimed by sea exploration. The expedition's overland presence strengthened American claims that helped lead to joint occupation under the Convention of 1818 and full American title under the Oregon Treaty of June 15, 1846. The expedition therefore accomplished exploration, science, diplomacy, commerce, and geopolitics in a single 28 month journey.