The Lewis and Clark Expedition, also known as the Corps of Discovery Expedition was the first American expedition to cross what is now the western portion of the United States, departing from St. Louis on the Mississippi River making their way westward through the continental divide to the Pacific coast.
The expedition took place shortly after the Louisiana Purchase in 1803, was commissioned by President Thomas Jefferson and consisted of a select group of U.S. Army volunteers under the command of Captain Meriwether Lewis and his close friend Second Lieutenant William Clark. The duration of their perilous journey lasted from May 1804 to September 1806. The primary objective was to explore and map the newly acquired territory and, by following the Missouri river northwest to see if it was connected or came close enough to the Columbia river, which flowed on to the Pacific Ocean. During this exploration Lewis and Clark aimed to find a practical route across the Western half of the continent, and in doing so, to establish an American presence in this territory thus securing it before Britain and other European powers tried to claim it. The campaign's secondary objectives were scientific and economical: to study the area's plants, animal life, geography, and other natural resources. During the course of the journey the expedition discovered many additional rivers and lakes along with numerous Indian tribes, many of whom they traded with and obtained useful information from. With maps, sketches and journals in hand, Lewis, Clark, and the other members of the expedition returned to St. Louis in September 1806 to report their findings to Jefferson.