Twice the Corps of Discovery had to cross the Bitterroot Mountains. These mountains, part of the larger Rocky Mountain range, form a portion of the border between the present-day states of Idaho and Montana.

In September of 1805, a Shoshone man, nicknamed "Old Toby" by Lewis and Clark, led the explorers on their first difficult, dangerous journey through the mountains. On September 14, 1805, Clark wrote in his journey of the exhausting trip:

"9 miles over a high mountain steep & almost inaxcessible much falling timber which fatigues our men & horses exceedingly, in stepping over so great a number of logs added to the steep assents and [descents] of the mountains . . . rained and snowed & hailed the greater part of the day all wet and cold"

	The Bitterroot Mountains, Lee Metcalf National Wildlife Refuge

In June of 1806, the Corps, now on their return journey, was anxiously waiting for the snow to clear so they could cross the Bitterroots a second time. This time, they were staying with a band of Nez Perce, who had warned them that the mountains were still impassable. Too impatient to wait any longer, the expedition started into the mountains on June 15. Although the beginning of summer, they found that the snow was still 12 to 15 feet deep. Knowing they could not make the trip alone, Lewis and Clark sent two men back to the Nez Perce to see if they would be willing to guide the Corps through the mountains. After several days, the men returned with three Nez Perce guides, who led them safely through the mountains in just six days.

Before entering the mountains, they stayed at a camp in the Bitterroot Valley that they called Travelers' Rest. This area had been used for many years by the Native Americans as a campsite. The Corps stayed at Travelers' Rest on their return journey as well, making this site one of the landmarks of the expedition. Today, most of the campsite area is a state park, and archaeologists have discovered several pieces of evidence left behind by the Corps of Discovery.