The most important transcontinental journey in US history. The expedition was commissioned by President Jefferson to explore the vast area acquired as a result of the Louisiana Purchase. Commanded by Meriwether Lewis and William Clark, it left St Louis in 1804 and sailed up the Missouri to winter in North Dakota before crossing Montana to the foothills of the Rockies. Crossing the Continental Divide at Lemhi Pass, Idaho, the expedition then moved north and in November 1805 reached the Pacific via the Clearwater, Snake, and Columbia rivers. In 1806, after crossing the Rockies, the expedition returned.