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
Geology of the Grand Canyon area relatively more important than they were before. Steeper cliffs and further widening the Grand Canyon and its tributary canyon system occurred. An average of two debris flows per year reach the Colorado River from tributary canyons to form or expand rapids. This type of mass wasting is the main way the smaller and steeper side canyons transport sediment but it also plays a major role in excavating the larger canyons. In 1963 Glen Canyon Dam and other dams farther upstream started to regulate the flow of the Colorado River through Grand Canyon. Pre-dam but still historic flows of the Colorado