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
Flora of the Arizona Upland Flora of the Arizona Upland Flora of the Arizona Upland' includes higher elevation Sonora Desert plants that require more moisture and cooler climates than those of the adjacent Sonoran Desert areas in the Colorado Desert of the lower Colorado River valley area, and which can withstand frost, unlike plants of the Sonoran Desert south of the border between the United States and Mexico. The Arizona Upland receives between 12 and 15 inches of annual precipitation, of which half falls in the summer monsoons. Cold air collects in the valleys. Less cold tolerant species grow on the warmer slopes above. The