The Lewis and Clark Expedition or the Corps of Discovery Expedition was the first American expedition to cross what is now the western portion of the United States. It began near St. Louis, made its way westward, and passed through the continental divide to reach the Pacific coast. The campaign''s secondary objectives were scientific and economic: to study the area''s plants, animal life, and geography, and to establish trade with local Native American tribes. With maps, sketches, and journals in hand, the expedition returned to St. Louis to report its findings to Jefferson.