The three-mile long river passage, now a reservoir, cut through 1200-foot high cliffs. Lewis' journal of July 19, 1805, describes the passage and the optical illusions common to travelers: "the towering and projecting rocks in many places seem ready to tumble in on us...his rock is a black granite below and appears to be of a much lighter color above. This extraordinary range of rocks we called the Gates of the Rocky Mountains."
The towering cliffs and twisting passage earned the name as passage through the canyon gave the illusion that the gates swung wide to allow travel. Lewis was so hopeful of meeting with signs of local tribes that he made careful notes of anything unusual.
His notes referred to "big-horned anamals," meaning bighorn sheep, traces of Indian camps, the particulars of plant life, and the merits of dried buffalo dung as fuel for cooking fires. His descriptions of the passage are the most colorful and capture his mood:
"...the rocks approach the river on both sides, forming a most sublime and extraordinary spectacle. Nothing can be imagined more tremendous than the frowning darkness of these rocks, which project over the river and menace us with destruction."