I completed a map of the Countrey through which we have been passing from the Mississippi at the mouth of the Missouri to this place. In the Map the Missouri, Jefferson's river, the S.E. branch of the Columbia or Lewis's river, Koos-koos-ke and Columbia, from the enterance of the S. E. fork to the pacific Ocian, as well as a part of Clark's river and our track across the Rocky Mountains, are laid down by celestial observations and Survey. . . . We now discover that we have found the most practicable and navigable passage across the Continent of North America.
William Clark, the expedition's cartographer, drew sketch maps en route, and assembled them into three comprehensive maps of the Western United States and lower Canada, reflecting successive changes in the way the West was envisioned. The first was drawn at Fort Mandan in 1804-05, the second at Fort Clatsop in 1805-06, and the third – following the expedition – in St. Louis in 1809-10.
His third map, revised and published in 1814, showed that the western half of the continent was not a mirror of the East, and that the Rocky Mountains did not resemble the Appalachians or other eastern ranges, as had been supposed, but consisted of a large number of interlocking ranges nearly 250 miles wide in the area where Lewis and Clark crossed them. It defined the shape and scope of the Missouri and Columbia River drainages. Above all, it permanently disposed of the durable old legend of a continuous, or at least near-continuous, navigable water route across North America.
Clark's final map served as a basic reference and guide to westward-bound Americans for another three decades, all the while being steadily revised and expanded by fur traders, missionaries, emigrants, and entrepreneurs on the Santa Fe Trail (1821), the Mormon Trail (1840) and the Oregon Trail (1843). Further government surveys between 1850 and 1890, plotting routes for wagon roads and railroads, added still more dimensions to Americans' understanding of their nation's geography.
The construction of modern highways began under the impetus of bicyclists in the East in the 1880s and 90s, and the movement toward a national highway system continued with the Good Roads Movement around the turn of the twentieth century. Spurred by the rapid growth of the automobile industry after 1900, the Federal Aid Highway Act of 1921 established state highway systems, and led to the designation of cross-country federal highways such as U.S. 2, 10, 12, 14, and 30, portions of which more or less closely parallel the route of the Lewis and Clark expedition. The Interstate Highway System, begun in the 1950s, once more changed travelers' perspectives on the West.
In the mid-twentieth century, the waterways that Lewis and Clark followed from home to the Pacific Ocean and back – the Ohio, Missouri, Jefferson, Beaverhead, Clearwater, Snake, and Columbia Rivers – were largely rebuilt, mile by mile, into a series of commercial waterways and reservoirs for irrigation and flood control.
Encyclopaedia Britannica CD 97, "Roads and Highways."
John Logan Allen, Lewis and Clark and the Image of the American Northwest. New York: Dover, 1975.