The two men agreed to lead the expedition together even though Lewis outranked Clark. Both men were well experienced in the wilderness and had served in Army campaigns against the Indians. To add to this, Clark had experience in mapmaking and Lewis had training in the study of plants and animals. In order to prepare for the journey Lewis spent time studying in Philadelphia during the summer of 1803. There he obtained the knowledge on how to classify plants and animals and how to determine geographical positions by following the stars. After that, he traveled to Pittsburgh and in late August left in a keelboat, navigating to Louisville, Kentucky where he joined by Clark who had been recruiting men from a nearby Army post. Lewis and Clark chose expert woodsmen and hunters who were used to manual labor and military discipline to join the expedition. One other man traveled with the group, Clark's African American manservant, York.
In December 1803, the group set up winter quarters at Camp Dubois, across the Mississippi River from St. Louis. During their stay at the camp, Lewis and Clark trained their men and were educated by fur traders and travelers about the areas the group would explore.
The expedition set out from Camp Dubois in the keelboat and two pirogues (dugout canoes) on May 14, 1904. They started out with fifty men; many were French boatmen temporarily hired to maneuver the weighty keelboat and other craft against the Missouri's rapid current. Aboard the keelboat they carried a huge amount of supplies, including food, medicine, scientific instruments, weapons, and presents for the Indians. .
While traveling up the Missouri River, the beauty of the land and its large amount of wildlife impressed Lewis and Clark. The group had a tense encounter with Sioux Indians in September. The Sioux refused to let Clark go back to the boat after the explorers had talked and traded gifts with them.
