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The towering Rocky Mountains represented the most physically challenging portion of the Oregon Trail. Travelers along the trail were forced to pass the Rocky Mountains in western Wyoming and Idaho, and then the Cascades Range in Oregon. Within the eastern Rockies, travelers along the Oregon Trail would pass over the Continental Divide, the "line" in which the directional flow of rivers changes. East of the Continental Divide, water flows out to the Atlantic Ocean or Gulf of Mexico, while west of the line, water flows to the Pacific Ocean.