Walking on water, I stride across the Yukon River where, for much of the year, choppy rips challenge canoeists and kayakers. There are no waves in winter.

Today the temperature is minus 10 degrees, blasted colder still by stinging gusts from the Arctic.

“It’s unusually warm,” the locals tell me without irony. We stand together on the thick, featureless snow that blankets the ice covering the water. Below us, unseen, fish swim; life goes on. Here on the surface, the firm grip of winter holds everything in frigid suspension. Only the bundled people move. And the piercing wind.

It is a northern sky above us: pale, pink-tinged and cloudless. The glaring sun has, at last, curved above the frosted pines fringing the small city of Whitehorse. This is Canada’s Yukon Territory in the middle of February. The severe darkness of December and January has been relieved. My companions contend that this is the best time of year, with everything looking brand new under hard light and unblemished snow.

We are here to witness the start of an odyssey. In town, a dozen teams are counting down the minutes to the Yukon Quest International Sled Dog Race, the world’s toughest dogsled race. Ahead of them, 1,000 miles of unforgiving wilderness: two lonely weeks out in the cold.

“They’re a breed apart,”’ someone says.

“The dogs?”

“No. The mushers.”

And you can believe it. The dogs are bred for the hardships of the trail. If anything, minus 10 degrees is a little warm for them. Each dog is selected for its toughness, stamina and ability to work as part of a team. At this moment, they are straining on their harnesses, eager to get going.

But what of the people who will be out there with them? Why on earth would any man or woman voluntarily go through such an extreme trial of physical and mental fortitude?

At her log house outside Fairbanks, Alaska, I met with Mary Shields, who in 1974 became the first woman to complete the famous Iditarod Great Sled Race between Anchorage and Nome. She seemed normal enough. We walked down to the fenced area where she kept her dogs. Most of them were lying curled up on the snow with their tails covering their noses. A couple stood on the roofs of their kennels, watching our approach. It was minus 40.

Despite the fact that I was wearing thermal underwear, two pair of pants, two shirts, a jumper, a fleece, a winter coat, a hat and gloves, I was frozen. Shields was in her element. A breed apart indeed.

“How do you cope with the cold and the loneliness out in the wilderness?” I asked.

“It’s what I live for,” she said. “Nothing compares to the peace and beauty you find on the trail. Yes, it can get a little cold, but it’s not lonely. I’ve got the dogs. I couldn’t wish for better companions.”

The dogs greeted her, but they were less welcoming of me. I kept my distance, aware that the typical Alaskan Husky — or malamute — is part wolf. All of the dogs bore carefully chosen names.

“You pick names that you’ll be able to say when the wind chill is at its worst and your lips are numb.”

Shields has been mushing for 40 years. She happily welcomes visitors to her home to share her tales of the trail. She has faced some surprising challenges out in the Alaskan wilds.

“Sometimes you become convinced that you’ve got a passenger in the sled with you. That’s fine, if all you do is talk to them. But if you lean too far to compensate for their nonexistent weight while rounding a corner, you can turn the sled over. That’s dangerous, because the dogs sometimes keep on running, dragging away the sled with all your supplies.”

That nightmare must be on the minds of the mushers lined up behind the start line of the Yukon Quest. At two-minute intervals, one team at a time, they set off. They gallop along First Avenue, among cheering crowds, then slither down the bank to where several dozen of us stand on the river. In winter, the rivers and lakes become natural highways.

“The trick,” one musher told me, “is to know when to stop using them in spring. Mushers and dogs have died falling through the ice.”

With the changing global climate, mushers must be alert to the subtleties of their unforgiving environment. One miscalculation could be catastrophic.

Many mushers keep their teams fit and trained during the summer by running courses for tourists. It is a valuable chance for novices to learn the basics of mushing in relative comfort. Adapted sleds with wheels are usually used, though the principles remain the same.

The first thing you must learn is how to talk to the dogs. There are four crucial commands. “Mush” is go, “whoa” is stop, “haw” is left and “gee” is right. Although you shout these words for all the dogs to hear, in practice you are instructing the lead dog.

There is nothing quite like the exhilaration of traveling under dog-power. You rumble along at a surprising speed and can easily imagine yourself as one of the early pioneers striking out into the unknown wilds. If there is a drawback, it is the view. There is no getting away from the fact that when you are on the sled, you are gazing at the least attractive end of the dogs. It’s not a pretty sight.

The mushers on the Yukon Quest won’t be worrying about that. They slide by, human and canine eyes focused on the 1,000-mile trail ahead of them, two species working in harmony.

We watch each team slip into the distance and, though we wouldn’t want to be in their place, we can’t help feeling a pang of envy. They are embarking on one of the world’s last genuine adventures.

INFO TO GO

The two epic annual dogsled races of Alaska and the Yukon do not make for natural spectator sports. Once out on the trail, the teams are often many miles apart. But the starts are well worth witnessing. The Iditarod Great Sled Race (400 D St., Anchorage, tel 907 376 5155, http://www.iditarod.com) departs from Anchorage on the first Saturday in March. The Yukon Quest International Sled Dog Race (1109 1st Ave., Whitehorse, Yukon, tel 867 668 4711, http://www.yukon quest.com) takes place in February, starting from Whitehorse in odd years and from Fairbanks in even years. Guided tours for both races, including the chance to experience mushing firsthand, are available from Alaska Tours (413 G St., Anchorage, tel 866 317 3325 or 907 277 3000, http://www.alaskatours.com).

The oldest dogsled event in the United States is the American Dog Derby (tel 208 652 7980, http://www.americandogderby.org), actually five different races held in Ashton, Idaho, in February. You can arrange a visit with the legendary Mary Shields in Fairbanks, Alaska, by visiting her Web site, http://www.maryshields.com The C.athers family (Box 33092, Whitehorse, Yukon Y1A 5Y5, tel 867 333 2186, http://www.cathersadventures.com) run participatory dogsledding tours from their idyllic home beside Lake Laberge near Whitehorse, Yukon. A four-day trip including instruction and accommodation in log cabins costs from $1,500 per person, departing from Whitehorse.