Final answer:
Lewis and Clark used the Missouri River and its tributaries to navigate through the Louisiana Purchase and Oregon Country during their expedition as they searched for an all-water route to the Pacific Coast.
Step-by-step explanation:
The natural feature that Lewis and Clark utilized extensively during their expedition through the Louisiana Purchase and Oregon Country was the Missouri River and its tributaries. This river system served as the primary route for their journey towards the Pacific Coast as they sought an all-water route to further American trading interests. The expedition also involved mapping the territory, studying its natural resources, and fostering relations with Native American tribes. Despite the original intent, no direct Northwest Passage was found because such a route does not exist; however, their detailed maps and observations provided valuable insights into the vast Western territories of the United States.