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
Cottonwood Valley (Arizona and Nevada) Cottonwood Valley (Arizona and Nevada) Cottonwood Valley is a wide valley, on the Colorado River on the border between Mohave County, Arizona and Clark County, Nevada. It extends east and west from the river into both states and is the first wide valley south of the Black Canyon of the Colorado, the last in a series of great canyons the Colorado River passes through after leaving the Rocky Mountains on its way west and where it makes its turn to the south toward the Gulf of California. All of the Colorado River in Cottonwood Valley and the lower reach of