The Columbia could make traveling across the continent by water practical and safe...

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"...the mountains do not appear very high in any direction tho' the tops of some of them are partially covered in snow. This convinces me that we have ascended to a great hight since we have entered the rocky Mountains, yet the ascent has been so gradual along the vallies that it was scarcely perceptible by land. I do not believe that the world can furnish an example of a river running to the extent which the Missouri and Jefferson's rivers do through such a mountainous country and at the same time so navigable as they are. If the Columbia furnishes us such another example, a communication across the continent by water will be practicable and safe..."