What outcome did the Lewis and Clark expedition have?

The expedition's goal as stated by President Jefferson was "to explore the Missouri River, & such principal stream of it as, by its course & communication with the water of the Pacific ocean may offer the most direct & practicable water communication across this continent, for the purposes of commerce." In addition, the expedition was to learn more about the Northwest's natural resources, inhabitants, and possibilities for settlement.
Lewis' and Clark's journals of the expedition describe the natural resources and native peoples of the West and contain information on many scientific matters. Over the two year journey, the expedition had made more discoveries of landscapes, rivers, native cultures, zoology, and botany of North America than any scientific expedition. Their results/contributions were many and lasting.

One of the most significant contributions of the Lewis and Clark Expedition was a better perception of the geography of the Northwest and the production of the first accurate maps of the area. During the journey, Lewis and Clark prepared approximately 140 maps. One important result of the Lewis and Clark expedition was that it enabled the United States to lay claim to the Oregon region. This claim helped make possible the great pioneer movement that settled the West in the mid-1800's.
A second achievement of the expedition was a better understanding of the country's natural resources. During the journey, the expedition documented over 122 species of animals and approximately 178 plants. The expedition even sent a caged prairie dog, which had never been seen before in the East, to President Jefferson as a gift.
Another achievement of the Lewis and Clark Expedition was that it established friendly relations with some of the Native American tribes. For the most part, encounters between the three dozen Indian tribes and the expedition were successful. They collected a variety of Indian goods and gathered information on Indian languages and culture.