In the winter of 1983, the largest El Niño event on record, a series of "superstorms," battered the West. That spring, a massive snowmelt sent runoff racing down the Colorado River toward the Glen Canyon Dam. As the water filled the dam, worried federal officials desperately scrambled to avoid a dramatic dam failure. In the midst of this crisis, a trio of river guides secretly launched a small, hand-built wooden boat, a dory named the Emerald Mile,...