On the fourth of August, Lewis came to where the Jefferson forks into three streams. At first puzzled to know which to take, he decided to follow the middle one, and left the usual note to Clark on a pole at the junction. But when Clark arrived with his boats there was no pole, for being green, the beavers had carried it off; whereupon he ascended the northwest fork, not being able to judge so well as Lewis, who had the advantage of hill-top views. But the difficulties of passage up this rapid stream were so great, that after a day's rough travel Clark returned to the forks, there finding Lewis awaiting him. Naming the northwest fork Wisdom, and the southwest Philanthropy virtues which they ascribed to President Jefferson they regarded the middle stream as the Jefferson, and continued its ascent. Lewis kept on his way afoot, while Clark suffering from "the raging fury of a turner on my ankle muscle" followed with the craft.

The river now passed for much of the way under perpendicular cliffs of rocks, infested by rattlesnakes. The mountains were not high, yet covered with snow, showing that the altitude was great, although the ascent had been scarcely perceptible. "I do not believe,"writes Lewis, "that the world can furnish an (other) example of a river running to the extent which the Missouri and Jefferson rivers do through such mountainous country, and at the same time so navigable as they are.

The following day (August 12th), Lewis reached the source of the Missouri a spring of ice-cold water "issuing from the base of a low mountain or hill." Two miles below this, "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." A little later in the day, the captain crossed the divide and reached "a handsome, bold, running creek of cold, clear water, where I first tasted the water of the great Columbia river"; this was the Lemhi, an upper tributary of the Columbia.

They thereupon struck off to the northward, seeking "the great river which lay in the plains beyond the mountains." The route taken was over the heavily timbered Bitterroot Mountains, which are slashed by deep gorges, down which rush torrential streams. This formidable region, "a perfect maze of bewildering ridges," was then and still is traversed by the Lolo or Northern Nez Perce trail, followed from time immemorial by Indians traveling between the upper waters of the Missouri and those of the Columbia.

Having left the region of game, the party were soon pressed for provisions, and were obliged to kill several of their horses for food. Blinding snowstorms in mid-September greatly impeded progress; the sides of the mountains were steep and rocky, with insecure foothold, especially during the frequent showers of sleet; the nights were cold, raw, and often wet; great areas strewn with fallen timber sometimes appeared almost impassable barriers; and not infrequently the rude path was dangerously near the edges of steep precipices, from which men or horses were in constant fear of being dashed to pieces. Thus they toiled on, through the dense and gloomy forests of pine, sometimes scaling steep ridges, at others descending rocky slopes at the peril of their lives, or threading the thick timber of marshy bottoms. Some of their horses fell through exhaustion, to be at once used as food; and the men themselves were so disheartened that Clark found it necessary to forge ahead with a party of hunters to find level country and game, by way of "reviving they spirits."

After safely braving the formidable Short Narrows of the Columbia "swelling, boiling & whorling in every direction" they passed camps of savages who were more familiar with white men, many of them being clad in civilized clothing obtained from the coast traders; if possible, these were even more tricky than their fellows above, and like them, dwelt in mortal fear of the Snakes and Shoshoni whom Lewis and Clark had met upon the sources of the river.

On the first of November they reached Pacific tidewater, and soon were amid rich bottom-lands and abundant elk, deer, and other game, among which were sea-otters; and dense fogs frequently veiled the pleasing landscape. On the fourth, the natives at one village came in state to see them, tricked out in scarlet and blue blankets, sailor jackets, overalls, shirts and hats, in addition to their usual costume assuming, disagreeable, thievish fellows, freely laying their hands on small things about the camp, but treated by the diplomatic explorers "with every attention and friendship." Three days later (the 7th), breakers could be heard during a storm, and Clark exultingly writes: "Great joy in camp we are in view of the ocean." The river was here from five to seven miles wide, with bold, rocky shores, and "The Seas ruled and tossed the Canoes in such a manner this evening that Several of our party were sea sick."

Finally, after being weather-bound for six days, in "a dismal niche scarcely largely to contain us, our baggage half a mile from us," and canoes weighted down with stones to prevent their dashing against the rocks, the wind lulled, they proceeded (November 15th) around a blustery point, and there found a "beautiful sand beech through which runs a Small river from the hills."

The continent had at last been spanned by American explorers.