Sah cah gah, we a our Indian woman verry Sick I blead her, we deturmined to assend the South fork, and one of us, Capt. Lewis or My self to go by land as far as the Snow mountains S. 20° W. and examine the river & Countrey Course & to be Certain of our assending the proper river, Capt Lewis inclines to go by land on this expedition, according Selects 4 men . . . to accompany him
– William Clark
If they found it, one landmark would cinch the argument—the Great Falls of the Missouri. To assure the doubting troopers that they were indeed on the right course, and to arrive quickly at a final resolution, Lewis decided to march overland. Pausing again to view the august if formidable spectacle of the Rocky Mountains, he passed over "a most beautiful and level plain" with "infinitely more buffaloe than I had ever before witnessed." In two days he arrived at the "sublimely grand specticle" of the falls.
Once again his poetic nature emerged. For two hours he sat on a rock overlooking the grandest sight he had ever beheld, and wrote at length of "the beauty of this majestically grand scenery." Even then he felt that he could not do justice to "this truly magnifficent and sublimely grand object, which has from the commencement of time been concealed from the view of civilized man." No small part of his rapture was derived from the certain knowledge that his decision at the Marias had been right.
Lewis arrived at the falls on June 13, 1805, anticipating an easy, one-day portage. The following day his initial amazement was compounded by the discovery of four more waterfalls. Then came the reality: The most direct portage route would be 18 miles long! Nearly one month of the Montana summer was to be occupied in transporting the canoes and equipment from Belt Creek, northeast of the falls, across rugged terrain to the upper portage camp east of three islands they named White Bear Islands. They were pummeled by storms of wind, rain and hail, tortured by prickly pear cactus, plagued by rattlesnakes and mosquitoes, and menaced by "white" bears—grizzlies. Captain Clark called it the most perilous and difficult part of their voyage. The only consolation was the abundance of buffalo nearby, ten thousand of them, at least.
The big white pirogue, too heavy to be portaged, was left behind. Anticipating such a circumstance, Lewis had designed and brought along a portable strap-iron framework for a boat that could be covered with animal hides and sealed with pine pitch. Unfortunately, there were no pine trees in the vicinity of the White Bear Islands, and the improvised caulking of pounded charcoal and buffalo tallow didn't work. The boat sank; Lewis's "experiment" was a failure, and another five days were spent hewing two cottonwood dugouts to replace it. After digging several caches to conceal some excess baggage, including specimens of the new plants Lewis had found along the Missouri, the Corps of Discovery got underway again on July 15. They were less than 100 miles upstream from the Marias River, which they had left on June 12.
We arrose very early this morning, assigned the canoes their loads and had it put on board. we now found our vessels eight in number all heavily laden, notwithstanding our several deposits; tho' it is true we have now a considerable stock of dryed meat and grease. we find it extreemly difficult to keep the baggage of many of our men within reasonable bounds; they will be adding bulky articles of but little use or value to them. At 10 A. M. we once more saw ourselves fairly under way much to my joy and I beleive that of every individual who compose the party. I walked on shore and killed 2 Elk near one of which the party halted and dined. we took the skins marrow bones and a part of the flesh of these Elk. in order to lighten the burthen of the canoes I continued my walk all the evening."
— Meriwether Lewis
The captains were worried. The expedition was quickly running out of time and space: time to cross the Rockies before winter, and space to find the Shoshonis, with their horses and guides. But another month of increasingly toilsome river travel would ensue before either was assured. Despite being the better boatman, Clark walked cross-country most of the time now, hoping to encounter the Indians before the guns of his hunters frightened them off. He saw distant smoke and other recent signs, but no Shoshonis.
Steadily but painfully, Lewis brought the main party upstream, past the tributaries he named for Secretary of the Navy Robert Smith and Secretary of War Henry Dearborn.
Navigating the gap he called the Gates of the Mountains, north of modern Helena, his description of the "remarkable clifts" matched his mental outlook.
"Every object here," he wrote, "wears a dark and gloomy aspect." His spirits brightened when Sacagawea began recognizing the country, and assured him that her people were not far ahead. Finally, on July 27, the river party joined Clark's advance guard at another essential point in the geography of the west, the Three Forks of the Missouri River. In twelve days they'd made 190 miles.
Both Capt. C. and myself corrisponded in opinon with rispect to the impropriety of calling either of these streams the Missouri and accordingly agreed to name them after the president of the United States and the Secretaries of the Treasury and state having previously named one river in honour of the Secretaries of War and Navy. In pursuance of this resolution we called the S. W. for, that which we meant to ascend, Jefferson's River in honor of Thomas Jefferson. the Middle fork we called Madison's River in honor of James Madison, and the S. E. Fork we called Gallitin's River in honor of Albert Gallatin.
Both Lewis and Clark reconnoitered the three rivers; their decision to pursue the Jefferson stemmed from the direction of its flow, and from Sacagawea's memories. Desperate, Lewis forged ahead with three picked companions, interpreter George Drouillard, Private John Shields, and Private Hugh McNeal. He was determined to find the Shoshoni.
Lewis continued up the Beaverhead River and turned west toward the divide at Horse Prairie Creek, south of today's Dillon, Montana. Marching up the gradual slope on the morning of August 11, with Drouillard and Shields as flankers, Lewis spotted a lone Indian on horseback, the first Indian they had seen since leaving Fort Mandan. Unfortunately, the flankers spooked the horseman, and he galloped off.
On the next day, Lewis reached the Continental Divide. McNeal had straddled the uppermost streamlet and thanked God he had "lived to bestride the mighty & heretofore deemed endless Missouri," and Lewis drank deeply from "the most distant fountain" that provided its waters.
at the distance of 4 miles further the road took us to the most distant fountain of the waters of the mighty Missouri in surch of which we have spent so many toilsome days and wristless nights. thus far I had accomplished one of those great objects on which my mind has been unalterably fixed for many years, judge then of the pleasure I felt in allying my thirst with this pure and ice cold water which issues from the base of a low mountain or hill of a gentle ascent... here I halted a few minutes and rested myself. two miles below McNeal had exultingly stood with a foot on each side of this little rivulet and thanked his god that he had lived to bestride the mighty & heretofore deemed endless Missouri. after refreshing ourselves we proceeded on to the top of the dividing ridge from which I discovered immence ranges of high mountains still to the West of us with their tops partially covered with snow."
— Meriwether Lewis
Descending steeply into the valley of the Lemhi River, one of the headwaters of the Columbia River, Lewis encountered a Shoshoni woman and two girls who led him to their village and introduced him to their chief, Cameahwait, who welcomed him cordially. Lewis persuaded the Shoshoni to accompany him back to the the expedition's "Camp Fortunate" on the Beaverhead River, where Clark and the main party were expected to arrive. There, after some tense moments of mutual apprehension and distrust, friendship was reconfirmed.