The Lewis and Clark Expedition had a very significant influence on the westward expansion of the United States. The meticulous attention to detail in journaling exercised by Meriweather Lewis and other participants in the expedition constituted an invaluable source of information on the physical geography, climates, and animal species of the vast landscapes that extended far beyond the original American colonies. President Thomas Jefferson, who had commissioned the expedition, was keenly interested in the territories acquired as part of the Louisiana Purchase, and the information acquired by the expedition’s members was instrumental in filling in the blanks left from the region’s previous French owners.
Westward expansion was not without its risks. Potentially hostile (in terms of their efforts at defending their ancestral tribal lands) indigenous tribes posed a near-constant threat, as did disease, harsh weather conditions, and enormous amounts of acreage yet-to-be developed for commercial and military purposes. Jefferson understood that expansion and consolidation of newly-acquired territories required in-depth knowledge of the terrain if significant numbers of pioneers were to reach and settle distant outposts. The Lewis and Clark Expedition provided that knowledge. The explorers scouted favorable routes for those who would follow and provided copious insights into the fauna/flora and wild game that would help to sustain migrants. They mapped out lakes, rivers, and streams and developed a vital sense of what paths would likely prove optimal. That the principal objective of the expedition was precisely the identification of such landmarks for eventual exploitation was established by President Jefferson in his June 20, 1803 correspondence to Lewis:
The object of your mission is to explore the Missouri river, & such principal stream of it, as, by it's course & communication with the water of the Pacific ocean may offer the most direct & practicable water communication across this continent, for the purposes of commerce.
That Jefferson fully intended the expedition’s data and insights to be employed for the purpose of westward expansion of the United States was further evident in his letter to Lewis:
Your observations are to be taken with great pains & accuracy to be entered distinctly, & intelligibly for others as well as yourself, to comprehend all the elements necessary, with the aid of the usual tables to fix the latitude & longitude of the places at which they were taken, & are to be rendered to the war office, for the purpose of having the calculations made concurrently by proper persons within the U.S.
Reading the journals of Meriweather Lewis, as well as those maintained by other members of the expedition, one cannot help but be impressed by the levels of detail provided. The journals were intended to be, and were in fact, a road map to westward expansion, and their significance cannot be exaggerated.