By June 1, 1805, the two expedition leaders, relying on information gathered from the Hidatsa Indians, estimated that the corps should be nearing the Great Falls in present-day Montana. Instead, they encountered an unexpected fork in the river, which the Hidatsa had neglected to mention.
June 3, 1805 [Capt. Meriwether Lewis]
This morning early we passed over and formed a camp on the point formed by the junction of the two large rivers....An interesting question was now to be determined; which of these rivers was the Missouri, or that river which the [Hidatsa] call...Missouri, and which they had discribed to us as approaching very near to the Columbia river. [To] mistake the stream at this period of the season, two months of the traveling season having now elapsed, and to ascend such stream to the rocky Mountain or perhaps much further before we could inform ourselves whether it did approach the Columbia or not, and then be obliged to return and take the other stream would not only loose us the whole of the season but would probably so dishearten the party that it might defeat the expedition altogether....an investigation of both streams was the first thing to be done....accordingly we dispatched two light canoes with three men in each up those streams.
June 8 [Sgt. Patrick Gass]
About 4 in the afternoon Captain Lewis and his party came to camp. They had been up the North branch about 60 miles, and found it navigable that distance; not so full of islands as the other branch and a greater quantity of timber near it and plenty of game, which is not the case on the South branch....The officers concluded that the south branch was the most proper to ascend, which they think is the Missouri. The other they called Maria's river.
June 9 [Lewis]
I indevoured to impress on the minds of the party all of whom except Capt. C. being still firm in the beleif that the N. Fork was the Missouri and that which we ought to take; they said very cheerfully that they were ready to follow us any wher we thought proper to direct but that they still thought that the other was the river and that they were affraid that the South fork would soon termineate in the mountains and leave us at a great distance from the Columbia.
On June 13, the expedition finally reached the Great Falls. Impossible to navigate, the 87-foot-high cataracts would force the explorers to carry their boats and supplies overland. Contrary to the Hidatsa's estimate of a half day for the overland trek, the nearly 17-mile portage would actually take 11 days.
June 16 [Lewis]
I now informed Capt. C. of my discoveries with rispect to the most proper side for our portage, and of it's great length, which I could not estimate at less than 16 miles....good or bad we must make the portage.
June 23 [Lewis]
[While portaging around the Great Falls, the men] are obliged to halt and rest frequently for a few minutes, at every halt these poor fellows tumble down and are so much fortiegued that many of them are asleep in an instant; in short their fatiegues are incredible; some are limping from the soreness of their feet, others faint and unable to stand for a few minutes, with heat and fatiegue, yet no one complains, all go with cheerfullness.