HOOVER DAM PLUGS the Colorado River like a giant art deco sink stopper, holding back Lake Mead and parceling out just enough water on the other side to create a friendlier, tamer Colorado River. Today I'll kayak that friendlier river, drifting through the great natural spectacle known as the Black Canyon Wilderness Area.

I meet my guide, Shawn Coleman—he's with a company called Boulder City Outfitters—in the sprawling parking lot of the Hacienda Hotel and Casino, where, at 6 a.m., crusty gamblers are still at it from the night before, smoking, drinking, and throwing quarters into the Wheel of Fortune slot machine. Shawn assures me it will be a relaxing paddle—the Colorado River below the dam is not the frightfest that it is about 200 miles upstream in the Grand Canyon.

"The lazy water lets you see more nature, look for petroglyphs, and hang out in hot springs," he says amiably.

We put in at the bottomless shadow of the dam, where more than a hundred workers lost their lives during construction in the 1930s. The paddling is easy. Sometimes we float right down the middle; sometimes we run our hands on the canyon wall as we quietly slip by. After a few hours, we drag onto a sandbar and explore a steamy man-made cavern called Sauna Cave, which was dug in 1931 as a potential geothermal power plant. This water still weeps from a crystalline back wall, and we sit for a while, taking deep breaths of the soggy, 120-degree air.

After that we paddle some more, the river's 54-degree water dripping on my lap as one arm reaches high to make the stroke and the other stays low. Shawn points at three desert bighorn sheep tiptoeing across a narrow ledge about two-thirds of the way up. We stash our kayaks on a different sandbar and start sloshing up a narrow slot canyon running with 98-degree water. Shawn is like a ballet dancer in his Tevas, scrambling over boulders that lie between us and where he wants to go. I follow him up a boulder and onto a ledge 15 feet above the river. That's when my Tevas fail me and I slide down the wall. I splash into a deep, aqua-blue pool—warmed by geothermal activity deep inside the rock—blinking and spitting out water. Shawn is a good sport, and when he sees me he jumps off the ledge and splashes into the pool, too, saying, "Let's soak here awhile."

The last three miles go fast, and the canyon opens up to a crepuscular dome of sky. Our last stop is a spot called Emerald Cave, where the sun's rays don't reach the water, but the water glows a vivid aquamarine, as if lit from below. Shawn points to the muddy work of cliff swallows on the rock above and to a red-tailed hawk that's angrily castigating a trespassing bird. We about-face and paddle back against the wind.