The Rocky Mountains divide rivers like the Columbia that drain into the Pacific Ocean from rivers like the Missouri that drain into the Atlantic Ocean.
Once across the Rockies, the expedition could use the Columbia. But to get from the Missouri to the Columbia they would need horses to carry their supplies over the mountains – a lot of horses.
In order to get the horses, the expedition would have to find and make friends with the Shoshone. They lived outside the Louisiana Purchase in what would come to be called Oregon Country.
