ON A FRIGID DAY IN JANUARY, POPE BARROW IS UPSIDE DOWN in the middle of the Potomac River, suffering from an excruciatingly cold head.

Viewed from the perspective of dry land and warm shoes, the situation is not encouraging. The water temperature is perhaps 39 degrees, the current powerful and swift. Suspended beneath his kayak, which now resembles a purple torpedo, Barrow is washing down river. Below him are more rapids and more rocks. Even in his nylon dry suit and life vest, a swim in such conditions would be memorable.

But Barrow is not about to abandon ship, which would mean yanking off his spray skirt and shedding his boat like a pair of blue jeans. Not yet, anyway. For the moment, Barrow is aware of just two things: air -- the lack of it -- and the vicious, aching cold that is working its way up his sinuses to lodge somewhere behind his eyeballs.

Somewhere in the numbed recesses of his brain, instinct starts to kick in where intellect has long since departed. Deliberately, automatically, Barrow begins to set up for an Eskimo roll, a maneuver named after those who developed it, which enables an inverted paddler to lever himself upright. Frozen fingers tighten around the varnished wooden shaft of his double-bladed paddle. He lays the paddle flat against the side of the boat, then swings it perpendicular to the hull with a broad sweeping motion. In the next instant, he unleashes a powerful burst of energy, rotating his torso downward and simultaneously snapping his hips. As the kayak rolls upright, his helmeted head is the last part of his body to leave the water. Barrow gratefully gulps a lungful of air, as do a handful of onlookers at river's edge.

A moment later, he paddles over to an eddy, unsnaps his spray skirt and dumps the water from his boat. "Boy," he says, in all apparent sincerity, "that was fun."

VIEWED FROM THE CONCRETE OBSERVATION DECK AT GREAT Falls Park in Virginia, the Potomac River is a sight to take the breath away. Water tumbles over the falls with a roar like a passing jet. Below, the river reassembles itself as a series of waves and rapids, boiling past vertical cliffs and enormous, knife-edged boulders. It's a forbidding, primeval landscape, a self-contained wilderness just 14 miles from the buildings and bustle of downtown Washington.

It is also one of the premier whitewater recreation areas in the eastern United States. Every spring and summer, local kayak paddlers cluster in the rapids below the falls like schools of small, bright fish. The sport is not for the faint-hearted. Twelve to 13 feet long, not much wider than a pair of hips, a kayak is worn as much as paddled. In expert hands, it's a match for almost any river. Stability, however, is not one of the kayak's strong points. Hence the need for the Eskimo roll, an all-or-nothing move that has spawned the kayak paddler's battle cry: "Roll up or die!"

Park visitors, gawking and pointing at the spectacle below, tend to view the paddlers as crazy daredevils, an impression reinforced by a prominent warning that seven people a year, on average, drown in the same stretch of river.

The sport would seem enough of a challenge without adding the risk of frostbite. Not surprisingly, most paddlers hang up their helmets when the snow starts to fly. But Pope Barrow, Mac Thornton and Gordon Bare are not among them. For this unlikely trio of government workers, boating season on the Potomac is a never-ending proposition.

They seem like such reasonable men. Barrow, 45, is an attorney with the Office of the Legislative Counsel of the House of Representatives, a graduate of Yale and Harvard Law School and a father of two. Thornton, 40, is a picture of scholarly reserve. Slender and bespectacled, he works as an attorney for the Department of Health and Human Services, suing doctors for Medicare and Medicaid fraud. Bare, 42, is a foreign affairs officer with the Arms Control and Disarmament Agency; by most reckonings, he has already acquired a lifetime of adventure, having served as a U.S. Army adviser in the Mekong Delta in 1969 and 1970.

Watching the trio play in the river, some of the appeal becomes apparent. Never mind that the tiny thermometer on Thornton's life vest reads 30 degrees, which hardly begins to describe the damp, penetrating chill of this overcast winter day. Today they are going surfing, kayak surfing. A half mile below Great Falls, at a place called Rocky Island, the river narrows and slides over a submerged rock shelf, creating an oddity that would drive a Southern California surfer nuts -- a wave that never moves.

One by one, they line up in the eddy adjacent to the standing wave with bows pointed upstream. Barrow goes first, windmilling his paddle furiously to keep from being swept down river. He slides onto the wave sideways, at a slight diagonal, ruddering the boat straight into the current. Then -- cowabunga! -- he is sashaying back and forth across the face of the wave, twisting his torso this way and that. His slender fiberglass and Kevlar kayak makes slapping sounds against the river, like a speedboat on a choppy lake, though he is going nowhere.

They pop on and off the wave, sometimes two or three at once: Bare is a blur of motion and speed, Thornton is lackadaisical as he slips gently onto the wave, then leans his weight forward to stay on the face. Though it never moves in relation to the river's edge, the wave surges and breaks in uneven rhythm. Bare, wearing a swimmer's nose clip and a helmet with an American flag, washes over the wave sideways, rolls onto his side and disappears in the foam. He braces himself up with a paddle blade and emerges a second later, sputtering and dripping ice water onto his rubber spray skirt.

After a similar immersion, Barrow rubs his bright red face to restore the circulation. "Cold enough for ya?" asks Thornton.

The river exerts a special pull on each person who paddles it. For Thornton, paddling seems to evoke an almost childlike appreciation of nature's capacity to awe and mystify. Here's how the 40-year-old attorney describes the joy -- there is no other word for it -- of surfing Rocky Island: "You're sitting there in one place, in the trough of the wave, and the river is at eye level, and it's like the whole river is coming right at you."

For Barrow, solitude is part of the appeal, especially in the wintertime. From the vantage point of his kayak in Mather Gorge, the Potomac seems wild and remote, impossibly far from his town house on Capitol Hill. It reminds him of rivers he has paddled in West Virginia's wildest mountain hollows, yet this is close enough that he can frequently get in an hour of surfing before breakfast.

Bare is a competitor, captivated by the athletic skills of a sport that demands grace and judgment as well as strength. He attacks the river with intensity, rarely smiling, his brow furrowed with concentration. A regular competitor in national whitewater races, he is also the assistant coach of the U.S. whitewater team, which trains year-round on the Potomac. Bare is on the river almost every day of the year.

"I hate the cold water," he confesses. "I just do it because I like the sport so much."

It may be, too, that there is a common denominator at work, that in a kayak each person finds a necessary escape that others might look for in a good book. Rapids concentrate the mind. "You're not worrying about tomorrow or yesterday or even the next rapid," Barrow says. "You're just thinking about what you need to do to get through this thing without cracking up your boat or washing up under a rock. It must be what a test pilot goes through, that sense of complete alertness and awareness."

Says Thornton, "It's a combination of being totally absorbed in what you're doing, and exhilarated at the same time. I can only tell you that it's almost a necessity in my life. If it were taken away from me, I would be extremely unhappy."

They are also aware that kayaking can be an obsession. "I had a friend who was a psychiatrist, a really maniacal paddler," Barrow recalls. "He was constantly challenging himself, running rivers of greater and greater difficulty, until it got to the point where the only thing he could do was at extreme risk to life." He quit the sport.

Nature is not the only challenge that paddlers face on the Potomac. Spurred by an alarming number of drownings in recent years, the National Park Service and the Maryland Department of Natural Resources have taken steps to regulate activities on the Potomac. Those steps, in Bare's view, intrude on an essential American birthright, the "freedom to risk your rear end."

The regular paddlers of the gorge are not part of the oft-discussed "drowning problem." That distinction belongs to the fishermen, sunbathers and swimmers who discover the Potomac's awesome power only when it is too late. Of 57 drownings in the Great Falls area between 1975 and 1985, only 17 were boaters, all of them novices, only a few wearing life jackets, according to a study commissioned by the park service. Alcohol or drug use is cited as a factor in more than half of all drownings in the Potomac.

But the debate continues. Under pressure from local legislators, who have suggested erecting plaques to mark the location of each death, authorities have banned paddling above several dangerous dams and prohibited the launching of boats above Great Falls itself. A recent park service notice said the river would be "closed" whenever its level rose above five feet, as it frequently does in springtime. The notice was amended only after Thornton and other local paddlers marched down to the Great Falls manager's office to complain. "Some waves you can't even surf until it hits five feet," says Barrow. Nonetheless, Thornton and other paddlers are among the most active safety advocates on the river, frequently collaborating with the park service on the design of warning notices and brochures.

Park Manager Bruce McKeeman says the park service follows an "assumed risk" policy -- the public must assume certain risks in a natural environment. In the view of the service, however, a wilderness area adjacent to an urban one poses a unique safety challenge. "What we have to do is balance what is safe use of the park with an individual's right to personal freedom and recreation."

ON A DAZZLINGLY BRIGHT, BITTERLY COLD SUNDAY IN January, a fresh layer of snow has transformed Great Falls Park into a winter paradise, and cross-country skiers are out in force. Over at the park office building, the food concession is doing a brisk trade in hot chocolate.

But the temperature has not deterred the faithful. One by one, Barrow, Thornton and Bare pick their way over snow-covered boulders to the river below. Ice forms a rim on the eddy where they deposit their kayaks. A hundred yards up river, ice floes the size of suitcases tumble over the falls.

The paddlers are creating quite a sensation. Dressed in brightly colored dry suits, green dish gloves and blue neoprene hoods, they look like refugees from a toxic waste cleanup site. A handful of Asian tourists, underdressed and shivering, stare in astonishment as the men wedge themselves into the cockpits of their kayaks and stroke out into the current. The visitors gasp and shake their heads as the river tosses the paddlers like bits of styrofoam.

Even in English, the spectacle would be difficult to explain. ::