The Lewis and Clark Expedition began in 1804, when President Thomas Jefferson tasked Meriwether Lewis with exploring lands west of the Mississippi River that comprised the Louisiana Purchase. Lewis chose William Clark as his co-leader for the mission. The excursion lasted over two years: Along the way they confronted harsh weather, unforgiving terrain, treacherous waters, injuries, starvation, disease and both friendly and hostile Native Americans. Nevertheless, the approximately 8,000-mile journey was deemed a huge success and provided new geographic, ecological and social information about previously uncharted areas of North America.