Sacagawea
On the trip back, they came near the Rocky Mountains in July 1806.  On July 6, Clark wrote "The Indian woman informed me that she had been in this plain frequently and knew it well.... She said we would discover a gap in the mountains in our direction..." which is now Gibbons Pass. A week later, on July 13, Sacagawea told Clark to cross into the Yellowstone River basin at what is now known as Bozeman Pass.  This was later chosen as the best route for the Northern Pacific Railway to cross the continental divide.