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
Glen Canyon Dam from Lee's Ferry. Because of the cold, clear water released from Lake Powell, the stretch of the Colorado River between Glen Canyon Dam and Lee's Ferry has become an excellent rainbow trout fishery. Trout are not native to the Colorado River system; they were stocked in the river below Glen Canyon Dam after the dam was built. Other non-native fish such as smallmouth bass, striped bass, largemouth bass and black crappie were planted in Lake Powell to provide sport fishing opportunities. Like many U.S. lakes and reservoirs, Lake Powell has an active problem with zebra and quagga mussels, invasive bivalve