THE LONE AFRICAN EXPLORER drags his kayak ashore and begins to collect firewood from around the little beach on the left bank of the White Nile. It's April 10, 2007, and the day's descent of some of the continent's most powerful rapids has worn him to exhaustion. But he can't sleep. Not without fire. He's also careful not to stray beyond the jungle's green curtain—this is Uganda's Murchison Falls National Park, after all, home to the world's densest populations of hippopotamus and Nile crocodile, one an extremely territorial 4,500-pound vegetarian with six-inch dagger tusks and the other a voracious 12-foot-long opportunist.

The explorer is Johannes Hendrik Coetzee, 32 years old, five feet eleven, with a thick build and a receding hairline shaved to skin. He's a former South African Defence Force medic and a giant in the world of whitewater exploration, having organized and led a historic source-to-sea descent of the Nile in 2004. Though he's charismatic and charming, the kind of guy who changes the gravity in any room he enters, he now prefers to travel alone. Four elite teams have descended Murchison's two-day section of Class V water before now, and Coetzee was on three of them. But nobody had ever tried it solo before this trip.

Now he sparks his fire in the quickening equatorial dusk, a lonely prick of light in a nearly 1,500-square-mile "chunk of untamed African savanna bisected by the mighty river Nile," as the park's literature proclaims. Below him, the river drops ferociously over a roughly 30-mile stretch before abruptly reaching the unrunnable 140-foot Murchison Falls itself, at the edge of the Rift Valley escarpment. The only humans this deep in the park are the rebels of the Lord's Resistance Army, which controls the right bank of the river and has, since 1987, been attempting to replace the Ugandan government with a strict Christian theocracy.

Hendri, as he's called, is an obsessive chronicler of his adventures. He takes mental notes that he'll later type into his laptop journal. Across the river, a big storm that's filling the sky is approaching. It's still far off and I sit and watch the lightning until it reaches me.

Barefoot as always, he feels vulnerable, but not afraid. I ask myself, Are you ready to die? I give it some serious thought. I believe I am. I look back on my life, and I feel satisfied.

He slides inside his bivy bag and falls asleep. During the night I am aware of the rain at times, of water inside the bivy. I'm a sitting snack for anything that likes meat, but I never fully wake until I feel the approach of dawn.

Coetzee unzips his bag, packs his boat, and slips back into the barreling 20-foot standing waves and terminal ledge holes. His Fluid-brand creekboat is seven feet three inches of plastic sealed with a neoprene skirt that locks over the cockpit rim and snugs around his rib cage. Coetzee balances through the tumult of 80-degree water in part with forward momentum, like a skier, but mostly by heaving his 195-pound frame, forehead tucked behind his leading elbow, into wall after wall of unrelenting chaos.

Then calm—but not safety. The eddies and banks are as dangerous as the rapids. The river is so narrow that I have to worry about the flat dogs on both banks.