As we now know, the stream that came in from the north was that which is still called Maria's (or Marais) River, and the so-called branch from the southwest was the Missouri River. Lewis and Clark, however, were in the dark as to the relations of the two streams. Which was the parent? Which was the branch? After pondering all the evidence that could be collected to bear on the important question, the two captains agreed that the southern stream was the true Missouri, and the northern stream was an important branch. The journal says:

"These observations, which satisfied our minds completely, we communicated to the party; but every one of them was of a contrary opinion. Much of their belief depended on Crusatte, an experienced waterman on the Missouri, who gave it as his decided judgment that the north fork was the genuine Missouri. The men, therefore, mentioned that, although they would most cheerfully follow us wherever we should direct, yet they were afraid that the south fork would soon terminate in the Rocky Mountains, and leave us at a great distance from the Columbia. In order that nothing might be omitted which could prevent our falling into an error, it was agreed that one of us should ascend the southern branch by land, until we reached either the falls or the mountains. In the meantime, in order to lighten our burdens as much as possible, we determined to deposit here one of the pirogues, and all the heavy baggage which we could possibly spare, as well as some provision, salt, powder, and tools. This would at once lighten the other boats, and give them the crew which had been employed on board the pirogue."