The first land route across what is now the United States was partially mapped by the Lewis and Clark Expedition between 1804 and 1806. Lewis and Clark initially believed they had found a practical overland route to the west coast; however, the two passes they found going through the Rocky Mountains, Lemhi Pass and Lolo Pass, turned out to be much too difficult for wagons to pass through without considerable road work. On the return trip in 1806 they traveled from the Columbia River to the Snake River and the Clearwater River over Lolo pass again. They then traveled overland up the Blackfoot River and crossed the Continental Divide at Lewis and Clark Pass and on to the head of the Missouri River. This was ultimately a shorter and faster route than the one they followed west. This route had the disadvantages of being much too rough for wagons and controlled by the Blackfoot Indians. Even though Lewis and Clark had only traveled a narrow portion of the upper Missouri River drainage and part of the Columbia River drainage, these were considered the two major rivers draining most of the Rocky Mountains, and the expedition confirmed that there was no easy route through the northern Rocky Mountains as Jefferson had hoped. Nonetheless, this famous expedition had mapped both the eastern and western river valleys (Platte and Snake Rivers) that bookend the route of the Oregon Trail (and other emigrant trails) across the continental divide—they just had not located the South Pass or some of the interconnecting valleys later used in the high country. They did show the way for the mountain men, who within a decade would find a better way across, even if it was not to be an easy way.