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
Course of the Colorado River Bridge carries U.S. Route 89 over the Colorado River. Below here the Colorado flows through the last of Glen Canyon, passing the scenic Horseshoe Bend, before emerging briefly from its canyons at Lee's Ferry, where it is joined by the warm, shallow, muddy Paria River. Historically a ford used by Native Americans, explorers and settlers, Lee's Ferry serves as the dividing point between the Upper and Lower Colorado River Basins, which were each allocated one-half of the Colorado River flow in the Colorado River Compact of 1922. Below Lee's Ferry the Colorado begins to trend more south than southwest, through