The Marias River Decision

The expedition faced perhaps its most critical navigational decision at the confluence of the Missouri and an uncharted river (which Lewis would name the Marias). The wrong choice could doom the expedition.

“An interesting question was now to be determined; which of these rivers was the Missouri. To mistake the stream at this period… would not only loose us the whole of this season but would probably so dishearten the party that it might defeat the expedition altogether.”

Nearly every member of the party believed the muddy, north-flowing fork was the Missouri. But Lewis and Clark, reasoning from geographic principles and the descriptions given by the Mandan, correctly identified the clear, south-flowing stream as the true Missouri. Their judgment was vindicated days later when they discovered the Great Falls.