Boulder Canyon (Colorado River) 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 under the eastern end of the Boulder Basin of Lake Mead, between Canyon Point in Nevada and Canyon Ridge in Arizona. Its original mouth is now underneath
Geology of the Grand Canyon area the Rocky Mountains in early Miocene time terminated in landlocked basins in Utah, Arizona and Nevada but there is no evidence for a major river. Around 18 million years ago, tensional forces started to thin and drop the region to the west, creating the Basin and Range Province. Basins (grabens) dropped down and mountain ranges (horsts) rose up between old and new north–south–trending faults. However, for reasons poorly understood, the beds of the Colorado Plateaus remained mostly horizontal through both events even as they were uplifted about in two pulses. The extreme western part of the canyon ends at one