What were the effects of the expedition of Lewis and Clark?

They learned more about the lands of the West, which ultimately spawned the idea of Manifest Destiny, that the US was destined to occupy the continent.
Considering the Lewis and Clark expedition, the fact that they were part of a scientific expedition was extremely important, especially during the Age of Enlightenment. The new knowledge they obtained about the Northwest's geography, natural resources, and native inhabitants sparked American interest in the west, and strengthened the nation's claim to the area. It also gave Americans a sense of pride at a time when they badly needed symbols of national triumph. Americans at this time realized that their new nation was still very much divided and relatively weak, and Lewis and Clark's adventure offered Americans proof that the men of their nation could accomplish great deeds amidst enormous hardships.

Also spread was the idea of 'Manifest Destiny' as Americans increasingly assumed a continental destiny. This opened many door for poor families to move west, and some to prosper throughout the late 19th century. However this also led to many Native Americans and earlier Hispanic settlers to be dispossessed, along with the decimation of great herds of buffalo that Lewis and Clark themselves saw and were amazed by.