Now, paddling a single-seat kayak on the Río Espolón, a comparatively placid tributary, I grew distracted by the passing vista of Patagonia’s forested valleys and snowy peaks. Ignoring the river’s ever-changing moods, I hit an unseen eddy line, teetered off-balance, and plunged headlong into the frigid water.

I have never understood the mathematics of fluid motion, but in two minutes I learned more about the raw power of 20,000 cubic feet per second than in a lifetime of scholarship. Wrenching my face skyward to breathe through the spume, I swung my legs downstream to avoid crushing my head on solid granite. For 500 yards, the surging current bumped me painfully over extruding rocks.

Ahead lay horror. Surging over a cap of boulders, the Espolón took an abrupt turn, forming swirling pools that culminated in a churning, vertical circle of water known as a hydraulic. Getting caught in one can be fatal: It traps the swimmer in an endless series of underwater spins. Experts advise bundling up the body to reduce its surface area, but the outcome is largely dictated by the water.

Interminable seconds passed. I tried to stifle my panic as I went under, holding on desperately to the air in my lungs. At last, my life vest kicked into play and I surged back to the surface, spluttering and shaken. I clambered slowly onto the bank, trembling with cold and fear, before a surge of relief took hold and I broke into a fatuous grin.

COLIN BARRACLOUGH