William Clark`s Map of the Lewis and Clark Trail

The Upper Missouri River

Westbound: Fort Mandan to Three Forks, 04/07/1805 through 07/27/1805.
Eastbound: Will need to be covered separately, as they divided into separate groups on the return trip.

On Google Maps.

Captain Clark's Map, with overlays added by myself.

During the winter at Fort Mandan, the captains received from the Hidatsas vital information on the course of the Missouri River awaiting upstream. The Hidatsas customarily sent hunting and raiding parties as far west as the Rocky Mountains. Having been told that the river would narrow and become more shallow as they approached its source, they sent the bulky keelboat back downstream to St. Louis before setting out, replacing it with six canoes built over the winter.

The scouting reports did not mention the Marias River, and reaching it presented the expedition with a major dilemma. President Jefferson had clearly instructed Lewis to follow the Missouri River to its source, in order to define the extent of the Louisiana Purchase. The turbulent and muddy right fork was characteristic of the Missouri River, while the waters of the left fork were tranquil and clear. Lewis reasoned that the left fork characterized waters coming from the nearby mountains, which would explain the relative lack of silt. The Hidatsas had said that following the Missouri would bring them to a series of great cataracts, and the selected course was proven to be the correct one when the Great Falls of the Missouri were reached a week and a half after passing by the Marias.

The Hidatsas had also told them that after reaching the first connected chain of mountains, they would continue about seventy five miles downstream and then the Missouri would branch into three forks. That is where they had captured the Shoshone woman Sacagawea five years earlier.