In 1803 President Thomas Jefferson selected his personal secretary, Captain Meriwether Lewis, to lead a voyage up the Missouri River to the Rockies, over the mountains, down the Columbia River to the Pacific Ocean, and back. Lewis and his partner, Captain William Clark, made the first map of the trans-Mississippi West, provided invaluable scientific data on the flora and fauna of the Louisiana Purchase territory, and established the American claim to Oregon, Washington, and Idaho.