Which River Is the Missouri? Early June 1805

You are advising Captains Meriwether Lewis and William Clark of the Corps of Discovery.

Travelling up the Missouri River towards the Rocky Mountains, the expedition has reached a place where two large rivers meet. Following the wrong branch could cost weeks the expedition cannot afford before winter.

Known information

Neither branch is clearly shown on any map the captains carry.
The north fork is muddy and has the same ‘boiling and rolling’ character as the Missouri they have followed so far.
The south fork is clearer and faster, with a bed of smooth, rounded stones, like rivers that flow out of mountains.
Hidatsa people told the expedition that the Missouri has great waterfalls further upstream, which the expedition has not yet seen.
Almost every member of the party, including the experienced boatman Pierre Cruzatte, believes the north fork is the Missouri.

Reveal what happened (spoilers: decide first)

Fact

What actually happened

Lewis and Clark chose the south fork. (Take the south fork)

The expedition arrived at the fork on 2 June 1805 and spent several days scouting both branches. The captains concluded the south fork was the Missouri, although most of the men still disagreed. On 13 June 1805 Lewis reached the Great Falls of the Missouri, confirming the choice. Lewis named the north fork the Marias River.

Important

Possible consequences · Analysis and speculation, not fact

Following the Marias would probably have led the expedition north and away from the route to the Shoshone, costing time before winter. How serious that would have been is a matter of interpretation.

History does not reveal what would definitely have happened under the other choice. Many factors could have changed the outcome.

Context · Why it was decided this way

The captains reasoned that a clear river with a stony bed was more likely to come from the mountains, where they hoped to meet the Shoshone people and obtain horses to cross the Rockies.

The men agreed to follow the captains despite their doubts, which historians often highlight as a sign of the trust within the expedition. The site is now known as Decision Point.