Preparation for the expedition was exhaustive, and Jefferson had his own territorial and economic motivations for the journey. He had acquired plenty of knowledge about the subject which was shared with Lewis. Lewis was directed by Jefferson to study various matters related to medicine, astronomy, navigation, geography, and mapping under his supervision. Other well-known experts assisted Lewis with his studies, and Lewis took to the preparations well.

The explorers prepared a keelboat created near Pittsburgh, Pennsylvania. It held various provisions and items for their journey. They traveled down the Ohio River to meet Clark near Louisville, Kentucky, in 1803. Their boat contained many items for trade with the native Indians. It included silver peace medals with Jefferson inscribed on them. These friendship medals were to be distributed to tribes they met. The boat also held weapons to display military prowess and protect the party, such as high-grade European weaponry, firearms, knives, metals, and the best mapping and cartography equipment. They also prepared medicines, gifts, flags, and all kinds of survival equipment.

Lewis and Clark were prepared as much as they could be. They had been influenced by the journey of Moncacht-Apé who was a Native American explorer from the Yazoo tribe near Mississippi. It is believed he made a journey in the early 1700s across North America. His story and itinerary were given to French explorer Antoine-Simon Le Page du Pratz, who then published this in his memoirs. In the 1750s. A book with Le Page’s book was published in 1763 and was used by European and American pioneers,

Jefferson had Le Page’s book with Moncacht-Apé’s itinerary, and Lewis also took a copy with him during the expedition of the Northwest. However, it was in no way a perfect guide as it did not contain information about getting through the Rocky Mountains. Lewis and Clark believed that it was possible to transport boats from Missouri’s headwaters flowing westward, which proved to be untrue.

The journey was influenced by Jefferson’s knowledge and readings of explorers such as Captain James Cook and his voyages to the Pacific Ocean. Jefferson also read about Alexander Mackenzie’s routes to the Pacific coast in British Columbia in 1793. Mackenzie’s writings influenced Jefferson’s need to find a way westwards as soon as possible. Jefferson knew from Mackenzie’s reports of the Brit’s intention of controlling the area through the Columbia River. His interest in the journey was based upon this fact.