The two southern routes don't cross the Rockies. The route through Flagstaff crosses the Colorado Plateau. It skirts south of the Rockies through La Junta following the Santa Fe trail...La Junta is named for the point on the old Spanish maps where the alternative routes of Santa Fe trail came to a junction. La Junta (literally "the junction") is east of Salida Colorado where the Arkansas River provides an exit ("salida" in Spanish) from the Rocky Mountains. It's the route Francis Parkman used and describes in his book The Oregon Trail...he never got very close to Oregon. The opium might have had something to do with that.

The southernmost route traverses the mountains of the Basin and Range physiographic region.