When the corp went through the Rocky Mountains, it definitely added much more time to their trip than they were expecting because they thought the Rockies would be similar to the much smaller Appalachians they were used to. The Pacific Northwest has lots of cliff-like costal areas unlike the Atlantic costal plains, which are very flat. The cliffs were new to the crew making them dangerous. While documenting a new species, Lewis almost lost his life, falling down a cliff.
The geography and climate of the west were a key factor in the length of the expedition. In the Northern Plains, there were many of the rivers that Lewis and Clark potentially planned on following in attempt to take an all water route to the Pacific. The High Plains likely slowed the crew down because of the extreme temperatures.
The journey of the Corp's of Discovery was extremely important for U.S. history. The crew introduced the country to lots of new species of plant and animal. The new territory was deemed to be useful for trade and settlement. The crew also made many peaceful relations with Native American tribes. With this, the country grew larger and more powerful.