The instant my 14-foot raft dips into a gnarly hydraulic on the left side of Rapid No. 5, I realize we won't make it. Before I can yell "High side!" to have my passenger balance the boat, it flips, and the maelstrom sucks me under. I tumble in the crosscurrents like laundry in a dryer, unable to tell up from down. An eternity later, I surface, sputtering. After assuring myself that my companion is alive—and clinging to the rubber hull—I help her pull the raft into an eddy to right it.

Below the confluence where the Colorado River merges with the Green River, a sign warns of hazards downstream. And indeed, four miles from the junction, the once-mild river throws itself with renewed vigor into whirlpools and over ledges, against house rocks and headwalls, battling confinement. Here, the Colorado's convulsions splinter driftwood and oars as effortlessly as they do the egos of overconfident boaters.

"For will any one dare to tell me that business is more entertaining than fooling among boats?" — Robert Louis Stevenson

We're part of a long tradition. Scouting Rapid No. 15 from shore, we visit an inscription pecked into a varnished boulder in 1891 by members of a luckless mining expedition: Camp # 7, HELL TO PAY, NO. 1 SUNK AND DOWN. I'd read in a guidebook that boat "No. 1" was half of the crew's miserable fleet of wooden tubs. A purported ledge of silver they were chasing turned out to be nothing but mica and schist—fools' dreams glittering in the sun.

But not all here is mayhem and waterlogged misery. Downstream from the whitewater gauntlet, not far from the calm of Lake Powell, forked tributaries breach Cataract Canyon's cliffs. Narrows with marine fossils embedded in smooth limestone boulders speak of a time when an ocean's tranquility blanketed this terrain. In a way, it still does—especially when the sun sets and the wind dies and the waning rapids murmur like surf on a distant shore.