Lost Trail. In early September 1805, Captain William Clark crossed the high mountains near today's Lost Trail Pass with part of the Corps of Discovery. Clark and their Shoshone guide, often called Old Toby, went ahead of the main group to look for a way toward the Columbia River. They had already learned from the Shoshone that the Salmon River canyon was too steep and rocky for people and horses to travel safely, so they did not actually try to float or follow the river very far. Instead, the narrow, twisting Salmon River later became known as the "River of No Return" because early boats could go down its strong current but could not easily come back upstream.

Since the Salmon River route was too dangerous, Clark's group had to cross a very steep and difficult mountain pass instead. On the Idaho side their guide lost the way, and the party ended up on rough hillsides and cliffs. As they tried to find the correct route back toward what is now Montana, their tired horses slipped and fell on the sharp, rocky slopes more than once. This hard and confusing journey is one reason the area became known as Lost Trail Pass.