The party set out early on September 1st, traveling cross-country over high, rugged hills, to today�s North Fork of the Salmon River (Fish Creek to Lewis and Clark), following the Shoshone guide, whom the captains called Old Toby. The were entering mountains more difficult to pass than any American had ever attempted. The country is so remote and rugged that nearly 2 centuries later it remains basically uninhabited.
Clark described the route: “thro� thickets in which we were obliged to Cut a road, over rocky hill Sides where our horses were in (perpetual) danger of Slipping to ther certain distruction and up and Down Steep hills…with the greatest of dificuelty risque &c. we made 7 1/2 miles.” But, as they ascended toward the Divide, the going grew worse. On September 3rd, it snowed. Clark summed up the misery of the day: “We passed over emence hils and Some of the worst roade that ever horses passed our horses frequently fell.” At least they got to the Divide (whether at Lost Trail Pass or Chief Joseph Pass is disputed), which they followed for some miles, along the present Idaho-Montana boarder, before beginning their descent to the Bitterroot Valley, west of the Divide.
For the next three weeks the expedition endured the very worse conditions while trying to cross the mountains. Snow, freezing cold, near starving, having to eat the 3 young colts they had with them until, finally, on September 21st, they found the Nez Perce� and obtained dried fish and roots to eat. It was one of the greatest forced marches in American history.
Although being forewarned, the party gorged themselves on the dried fish, roots, and berries and they almost all got violently ill, Lewis especially so. For over a week the expedition resembled a hospital ward for the critically ill more than a platoon of fighting men.