In the spring of 1983, a massive snowmelt sent runoff racing down the Colorado River toward the Glen Canyon Dam. Worried federal officials desperately scrambled to avoid a worst-case scenario: one of the most dramatic dam failures in history. 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, into the Colorado just below the dam’s base. The captain of the dory, Kenton Grua, aimed to use the flood as a hydraulic slingshot that would hurl him and two companions through 277 miles of some of the most ferocious white water in North America and, if everything went as planned, catapult the Emerald Mile into legend as the fastest boat ever propelled through the heart of the Grand Canyon.