Portion of the map British Possessions in North America . This map depicts geography, as known before the return of Lewis and Clark. The Missouri River is well shown to the Mandan Villages in modern North Dakota, and the Pacific coast is depicted in considerable detail. Original map in the private collection of J.S. Aber. These maps are representative of the geographic and ethnographic information available to Jefferson and Lewis during their initial planning for the journey.
Several ideas are noteworthy.
Once across the mountains, they also considered the trip downstream to the Pacific would be quick. The Rocky Mountains and Columbia River gorge turned out to be far more formidible obstacles than Lewis and Clark ever imagined prior to their journey. Jefferson set in motion the American empire with his decision. The future greatness of the United States rested upon Louisiana. Following the Louisiana Purchase, the Missouri River basin passed into the United States.