Answer:
Lewis and Clark traveled on the Missouri River until they found themselves at the Rocky Mountains (the canoes didn't work to well once they got there)... they traveled through Wyoming, Montana, and Washington till they reached the Pacific Ocean shore.
They found lots of unique and interesting fauna and flora of which they took samples and sent back to Washington D.C. They did not find a direct waterway from the Atlantic coast to the Pacific (which was one of their goals), but the expedition was a success overall.