Thomas Jefferson assumed North America had symmetrical geography, and mountains west of the Mississippi would be similar to the Appalachians
Source: National Oceanic and Atmospheric Administration (NOAA), GLOBE: A Gallery of Images - Color North America
The 200th anniversary commemorations of the exploration of Meriwether Lewis and William Clark highlighted the portion of their trip from St. Louis to the Pacific Ocean and back - but their journey started in Virginia, and finished in Virginia. Long before Lewis and Clark were sent on their expedition in 1803, political leaders in Virginia had recognized the benefits of exploring and claiming territory for Virginia based on the Second Charter issued by King James I in 1609.
Throughout the 1700's, Virginia's governors sought to establish English authority through the supposed "right of discovery," through conquest, or through treaty with Natives Americans. Excluding France and Spain from the Mississippi River watershed would provide international benefits for England, and local benefits for Virginians. In particular, land companies organized by the Virginia gentry (such as the Ohio Company and the Loyal Land Company) could generate personal profits for well-placed Virginia officials - once England gained control of western lands and Virginia, rather than Pennsylvania or anoher colony, established its authority to manage land sales west and south of the Ohio River.
After the American Revolution, far-seeing leaders also saw political risks for the new United States as settlers moved west of the Appalachians.
George Washington recognized the need to connect the population centers on the Atlantic coastline with the developing settlements in the Ohio and Mississippi River watersheds. He sought to build canals and economic links across the barrier of the Appalachians, to integrate those westerners into the political and economic life of the communities east of the Appalachians.
The Whiskey Rebellion revealed how limits to trade across the mountains affected political unity. Washington feared that westerners shipping goods down the Mississippi River could decide that it was in their interest to break away from the United States and form their own nation(s).
Another Virginian, Thomas Jefferson, sought to purchae Mobile and New Orleans in order to strengthen the economic and political connections between the East Coast and the settlers occupying lands in the Ohio, Tennessee, and Mississippi River valleys. Before concluding the Louisiana Purchase, Jefferson sent a group of explorers across the Mississippi River into what was technically Spanish territory. A voyage of discovery to the middle of the continent and to the Pacific Oacean coastline would allow the United States to strengthen its land claims by right of discovery, and clarify the value of such lands before proposing acquisition by conquest or purchase.
While Washington and Jefferson were visionary in their understanding of geopolitical strategy, others were insular in their perspective. The Virginia government at the start of the American Revolution, led by Governor Patrick Henry, focused on the threat of British attack in the Chesapeake Bay area. George Rogers Clark made a request in 1776 for supplying the Kentucky settlers with gunpowder to defend against raids by Indian allies of the British forces based in Detroit. The leaders in the new Williamsburg were reluctant to send resources to the western side of the state.
Clark expressed the choice of the frontier settlers effectively - Virginia had to support the settlements if the settlers were to stay loyal. Otherwise, "if a country were not worth protecting, it was not worth claiming." The leaders in the Virginia Convention could expect the Kentucky settlements to separate from Virginia, unless Virginia committed scarce gunpowder to protect the frontier.1
The Virginia Convention knew British fleets and armies were a far greater threat in the Chesapeake Bay area, but finally agreed that the western settlements were important too.
Clark was the older brother (by 18 years) of William Clark.
Sergeant Charles Floyd (the one member of the Lewis and Clark expedition who died on the journey) was buried on the Missouri River on August 20, 1804, on a bluff overlooking what is now Floyd River at Sioux City, Iowa
Source: Smithsonian Institution, Catlin Virtual Exhibition, Floyd's Grave, Where Lewis and Clark Buried Sergeant Floyd in 1804 and ESRI, ArcGIS Online
1. Lowell Hayes Harrison, George Rogers Clark and the War in the West, University Press of Kentucky, 2001, p.9, https://books.google.com/books?id=hKsQ7yKYkaoC (last checked November 27, 2015)