THE EXPLORATION OF WESTERN
No great physical obstacles hinder movement westward from the Mississippi until the barrier of the Rocky Mountains is reached. There is only a gradual and almost imperceptible rise towards the west, and no forests delay the rapid movement of the traveller. The direction of the natural drainage was the most important factor in the history of westward movement to the Rockies. The rivers flow in parallel courses from the west to the east before they join the Mississippi. Explorers and traders followed the courses of these rivers, because they provided a supply both of water and of fuel, the latter being obtained from the cottonwood trees, which grew along their banks.
There are many eastward flowing rivers, but the Missouri was by far the most important, as it was the only river on which navigation was always possible. The other rivers either possessed so little water that they entirely evaporated in the plains, or in some cases were only navigable for short periods of the year.
The superior navigability of the Missouri resulted in the early exploration of this river. The Missouri was already known as far as the villages of the Mandans, when Lewis and Clark pushed their way up the river in 1804. The fur-traders who followed Lewis and Clark explored the sources of the Missouri, from the source of the Big Horn in the south, to the source of the Milk River in the north. It seems natural that the earliest explorations of the west of the present United States should have been confined to the sources of the Missouri.