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Jefferson directed the captains to proceed up the Missouri River, note the tributaries, establish accurate maps of the region, and identify important locations for economic activity. He was explicit: “The point of your mission is to explore the Missouri river, & such principal stream of it, as, by it’s course and communication with the waters of the Pacific ocean, whether the Columbia, Oregan, Colorado or any other river may offer the most direct & practicable water communication across this continent for the purposes of commerce.” In his letter of instructions to Meriwether Lewis, national expansion and economic development dominated Jefferson’s thinking. In the meantime, Lewis had enlisted William Clark, experienced frontier army soldier and younger brother of Revolutionary War hero George Rogers Clark, to join him as co-captain of the Corps of Discovery. By late June 1803, Jefferson had formulated a letter of instruction that outlined an ambitious exploration agenda. Lewis perused Jefferson’s library and discussed the western regions with him, especially after the president decided to launch the expedition.
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A fellow Virginian and son of a plantation owner near Monticello, Lewis was a captain in the army with extensive knowledge of military discipline, experience on the western frontier, and as Jefferson put it “habituated to the woods, & familiar with Indian manners & character.” Lewis had served Jefferson since early in 1801, when the president had selected him as his private secretary to aid in the president’s plan to reduce the size of the regular army. Months before his message to Congress, the president had chosen Meriwether Lewisto lead the western expedition.