Boulder Canyon (Colorado River) steamboats as they passed over the dangerous rapids of Devil's Gate Canyon, during their high water runs to the mouth of the Virgin River at Rioville, Nevada. Boulder Canyon (Colorado River) Boulder Canyon, originally Devils Gate Canyon, is a canyon on the Colorado River, above Hoover Dam, now flooded by Lake Mead. It lies between Clark County, Nevada and Mohave County, Arizona. It heads at western end of the Virgin River Basin of Lake Mead, at about . Boulder Canyon divides the Black Mountains into the Black Mountains of Arizona, and the Black Mountains of Nevada. Its mouth is now
Black Canyon of the Gunnison National Park into Gunnison Gorge National Conservation Area. The canyon's name owes itself to the fact that parts of the gorge only receive 33 minutes of sunlight a day, according to "Images of America: The Black Canyon of the Gunnison". In the book, author Duane Vandenbusche states, "Several canyons of the American West are longer and some are deeper, but none combines the depth, sheerness, narrowness, darkness, and dread of the Black Canyon." The Gunnison River drops an average of through the entire canyon, making it the 5th steepest mountain descent in North America. By comparison, the Colorado River drops an average