It’s difficult to fathom how the Colorado River could possibly carve the mile-deep chasm that is the Grand Canyon. But if one thinks of the river as a flume of liquid sandpaper rubbing the land over millions of years, it begins to make sense. “The finest workers in stone are not copper or steel tools,” Henry David Thoreau wrote, “but the gentle touches of air and water working at their leisure with a liberal allowance of time.”
In 1963, humans stopped time, when the brand new Glen Canyon Dam on the Utah-Arizona border cut off the reddish sediment that naturally eroded the Grand Canyon. Today the river runs vodka clear from the base of the dam....