1. Ft. Mandan
St. Louis

2. June 1, 1805:
After wintering at the Mandan villages, the expedition has made its way up the Missouri River, and they’re confident they’re on the right course. They’ve passed tributaries described to them by Native Americans they’ve met along the way.
But today they encounter a fork in the river, and it’s not apparent which river is the Missouri. None of their informants or their maps have indicated a tributary at this point. Their mission is to follow the Missouri all the way to its source.
Which fork should they take—the left (south) or the right (north) fork—to continue along the Missouri?

3. Facts to Consider: Left (south) fork of river: 372 yards wide
swifter and more shallow than the right fork
runs clear; surface is smooth and unrippled
riverbed consists of round, flat, smooth stones
less like the lower Missouri River
Right (north) fork of river:
200 yards wide
slower and deeper than the left fork
runs whitish brown;
is boiling and rolling
gravel bottom, mostly mud
more like the lower Missouri River

4. Other Factors to Consider
An Expedition member reports that Pierre Cruzatte, their most experienced boatman, who had “acquired the confidence of every individual of the party declared it as his opinion that the northern fork was the most true genuine Missouri and could be no other.”
Most men agree with Cruzatte.
Lewis and Clark have set their course to follow a mountainous route, and know that rivers run more clearly in the mountains than in lower regions.
Meriwether Lewis—June 9, 1805
“ the information [from Native Americans]
also argued strongly in favor of the south fork.
They informed us that the water of the Missouri
was nearly transparent at the Great Falls . . .”

5. “I think the crew should go________.”
Left (south)
Right (north)

6. Solution
Obstacle

7. Consequences
You Decide to Take the Right Fork While the majority of the expedition camps at the fork of the rivers, you, Lewis, and several others hike along the river that forks to the right (north). After about four days, Lewis writes, “I now became well convinced that this branch of the Missouri had its direction too much to the north for our rout to the Pacific.” You return to the expedition campsite, where Clark has returned from exploring the left fork of the river, but can’t be certain it is the Missouri. You wonder—did Lewis stop exploring too soon? Perhaps the river eventually turned south, or became wider. You, like most of the group, think the expedition should take the north fork. You argue so persuasively that the captains agree with you. Consequences: Oh, dear!
You Decide to Take the Left Fork
William Clark decides to explore the left (south) fork of the river. In the meantime, Lewis will explore the right-hand (north) fork. While the majority of the expedition camps at the fork of the rivers, you,
Clark, and several others hiked about 55 miles, the distance he and Lewis agreed they would explore. But you and Clark still aren’t sure the river is the Missouri.
You return to the expedition campsite, where Lewis has returned from exploring the right fork of the river. He has become fairly certain the river to the north is not the Missouri River. You trust Lewis’s judgment, and you think the expedition should take the left fork.
Congratulations! You chose the left fork—the Missouri River!
The north fork is the Marias River, which eventually turns west, and
ends soon after that. If the expedition had followed your advice, they
would have had to backtrack, losing precious summer travel days.
Certainly, the group’s morale would have fallen. But it’s possible that
the expedition would have had to wait until the following spring to
cross the Rocky Mountains.

8. Ft. Mandan
St. Louis