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
Floods in the United States: 1901–2000 which was recorded at on July 1. The Colorado, which is usually dry at the mouth due to extensive diversions, reached the Gulf of California for the first time since the 1960s. Due to a spillway design error at Glen Canyon coupled with the relentless flooding, the dam at Glen Canyon very nearly failed, which would have emptied Lake Powell (the second-largest man-made lake in the US) and flooded out the entire lower valley of the Colorado River, cutting off water supplies to millions of people. By August as the water finally began to recede, the dam held, though only