Hugh and Betty Wilcox, 72 and 69 years old respectively, hadn't really planned on a swim.

The retired botany professor and his wife are an experienced team canoeing on flat water, but yesterday was their first day in white water. Now they are bobbing fully clothed in the Rio Grande, hanging onto a 17-foot Old Town canoe and whisking through a shallow rapid that divides Mexico and Texas.

This is the second day of an 83-mile, week-long journey through the Rio Grande's lower canyons just east of Big Bend National Park. The expedition is at least 30 miles from the nearest border village, and much farther from anything you'd want to call civilization.

The river is dun-colored and opaque, full of microbes that, we've been warned, can cause a host of diseases. Fortunately, even though it's early March the water is reasonably warm.

"Swimming in the Rio Grande is not recommended," a National Park Service pamphlet warns. "The river can be hazardous, even in calm-looking water. Be aware of strong currents, shallow areas with sharp rocks and large tree limbs, and be watchful for trout lines with large hooks."

All of the Wilcoxes' personal gear and their share of the group's extensive provisions are afloat, tied loosely in the capsized canoe. Since their paddles have been carried off by the current, it is tempting to be at once literal and figurative -- Mr. and Mrs. Wilcox are up the river without a paddle.

Alas, this is not the worst of their luck. They paid an outfitter $800 each to go on the excursion and got stuck with a loud and coarse group of men who brought along too much tequila. I refer here to your correspondent and four companions: Chet, a professor of business from Rhode Island; Jay, his wayward younger brother, an airline pilot based in New Mexico; their much-abused, 69-year-old father Charlie, a geologist whose camouflage sun hat quickly earned him the moniker "Rambo of the Rest Home"; and Rob, a white-collar repo man doing business as a bank vice president in Maine.

By now it must seem to the long-suffering couple that they are trapped in some kind of a week-long fraternity reunion. Worse still, some of the guides have been reduced to the same boy's-camp behavior.

The Wilcoxes clamber up the bank. There are five days left, and there are almost a dozen major rapids between here and the waiting vans.

I have been canoeing in the wilderness with reprobates before -- these very ones. I'm finding it is only in the company of strangers who remind me of my parents that they embarrass me.

This is my first canoe trip south of Maine or Canada, and my first with guides. At this moment I'm busy making sure the Wilcoxes will have a souvenir photo of their white-water baptism.

Having spent much of the last 15 years looking through the lens of a camera loaded with black-and-white film, I've developed an affinity for contrast. And for a camera-toting canoeist, it would be hard to come up with a better source of contrasts than the canyons of the Rio Grande.

Separating Texas's Big Bend country and the Coahuila state of Mexico, the river carves spectacular limestone canyons up to 2,000 feet deep in the middle of parched, high deserts. Amid hundreds of square miles of dry dust, prickly pear cactus and gnarled mesquite, a ribbon of green cane-bushes lines the muddy banks. Contrasting springtime temperatures can exceed 100 degrees at mid-afternoon, and occasionally dip below freezing at night.

Placid stretches of river paddling are punctuated by sharp drops and racing white water. And, in contrast to canoeing almost anywhere else in North America, a springtime trip here offers wilderness paddling without the need for insect repellent or rain gear.

A guidebook to the lower canyons warns: "This is not a family or a beginner's trip. If you are not reasonably comfortable out-of-doors you will be miserable." A paddler's first view of the river, regardless of his predisposition toward the outdoors, is not pretty.

The lower canyons trip begins at La Linda, a border village in Mexico that owes its existence to Teflon. Dow Chemical formerly mined fluor spar (a raw material used in making Teflon) here, and the industrial complex still hulks over the river on the U.S. side.

Back at the orientation briefing in Odessa the night before, one of our guides described La Linda as "the last place you'd want to spend five minutes ... " And a printed river guide says, "The Mexican customs officials there will allow you to cross the bridge and use a dirt road on the Mexican side of the river ... It is not recommended that you leave your vehicle at La Linda any longer than necessary."

The uncomfortable ride from Odessa to La Linda in our outfitters' van helped condition us for the squalor of the border crossing. As we left our motel, one van wouldn't start and the other had a flat tire on its canoe trailer.

The guides, ever-resourceful, jiggled the distributor cap in the balky vehicle and got it started -- good enough to get us into the desert wilderness, and it would be more than a week before we had to worry about getting back out.

We stopped at one place for air, another to save money on gas. The guides placed bets on whether the leader of our expedition would miss the turn in Fort Stockton. "This makes seven years in a row," one of the guides commented dryly as our leader executed a U-turn moments later. Another confidence builder from the man who would lead us into what his brochure calls "one of the nation's last frontiers."

The drive to Big Bend National Park and then down to the river took most of the day.

Laboring up a long grade in the Glass Mountains, someone asked how old the van was. "1976 through '78, depending on which part you're talking about," a guide replied. The smell of burning oil was thick.

An hour after checking in at the Persimmon Gap ranger station inside Big Bend Park, we're on a mudflat at La Linda, unloading and divvying gear under the watchful eye of a young Mexican on a big chestnut horse. Split into two parties, we finally head downriver in the late afternoon.

Suddenly the two long days getting here don't matter anymore. The dry, desert air quickly clears oil fumes from the sinuses, and almost immediately the canoes enter a spectacular canyon.

Paddling solo, I immediately fall behind the pack, fumbling with cameras to get the first best photos of the dramatic terrain in the late afternoon light.

Our first day on the river is short. We paddle just long enough to leave the other group behind. Setting up camp amounts to unpacking the canoes, unrolling a tent fly and laying sleeping bags and pads on top. Tents are provided, but with no bugs and virtually no rain there is no need to set them up.

Jeremy, our "Sherpa" (a job title and an apt job description used widely in the guiding industry), carries a blackened, corrugated steel fire-pan in his canoe. It is required for open fires to spare campsites ugly burn-marks along this officially designated Wild and Scenic River.

The guides take care of all the cooking on this trip. Dinners are good, honest food -- shish kebab, sole with almonds, chili -- usually accompanied by plenty of carbohydrates and followed by a simple dessert.

At 8:45 p.m. we are lying on our bedrolls under the stars, listening to the river, savoring aromatic mesquite smoke from the cooking fire. The full moon provides enough light to make notes in a journal.

We encounter no major rapids during our first two days on the river, but some easy white water along the way gives the guides a chance to teach basic skills and to size up their charges.

Hot Springs Rapid, at the end of the third day, is the first test. We will scout several rapids each day for the rest of the trip. Most of them consist of boulders dumped into the channel by floods that periodically roar down numerous side canyons, depositing enough debris to make navigation interesting and challenging.

Hot Springs Rapid is a short but steep and obstructed pitch, unrunnable at some water levels. It requires both a good set-up and a little maneuvering on the way down. The guides insist that we carry the gear to the campsite and then put on helmets before they'll let us run the drop in empty boats. No one has any trouble.

We carry three days' supply of fresh water, which is surprisingly heavy, particularly when added to the weight of personal gear and the outfitter's elaborate system of coolers and food bags. Water is replenished at potable hot springs along the way, and the Hot Springs campsite is our first water hole.

While all the campsites have breathtaking scenery and reasonably good sleeping surfaces, this one, on the Mexican side of the river, also has a hot spring rocked up in a series of descending pools. The highest is for drinking water only; the lowest is for soaping and rinsing. Middle levels cook away the soreness from two days at the end of a paddle and one squashed in a smelly van.

There is a jeep road from the Hot Springs campsite up San Rosenda Canyon to a ranch 12 miles into Mexico. It's not uncommon to find Mexican goatherds in the area, and this is supposedly a spot where illegal immigrants cross, one of the guides reports. We see no one.

After climbing the talus slope and partway up the canyon wall, then spending an hour in the hot springs, we sleep out again, soundly, under the full moon and stars.

In the morning, we awaken to the strong fragrance of the desert in bloom. While still in my sleeping bag I watch a hummingbird visit the pale flowers of a blossoming mesquite tree, and a male cardinal flit through the scrubby vegetation.

On our way down the river we pause to scout a short drop. After much hand-wringing and strategy, the guides set up a throw-rope below and we take turns running the canoes through and congratulating one another.

Just as our last boat arrives safely, a 16-foot aluminum utility boat roars into view, coming down the river at full throttle. The fisherman at the wheel never even slows down approaching the falls, and has no problem riding the cushion of water over the drop. "Well, that kind of took the wind out of our sails," Charlie confesses.

Later in the afternoon, just before our fourth campsite, Upper Madison Falls fails the scouting test. It is a beautiful rapid with good navigable routes except for one tricky spot in the middle. A consensus emerges that it's unlikely seven out of seven boats can make the sharp turn required just above that spot.

Rather than carry the food, water, gear and canoes around the falls, we set up a relay team and use ropes to line the loaded boats through a side chute. Chet, ever the amphibious cowboy, body-surfs into an eddy after following the last boat down the run-out.

Despite their early misadventure, the Wilcoxes handle the canoe well under the coaching of our guides. Their close encounter with the current turns out to be the only time anyone dumps during our week on the river.

Just below the falls we camp in the shadow of Burro Bluffs, a 1,000-foot sheer wall separated from the river by a bank wide enough to camp on. Each day I've made an effort to climb into the desert to explore, and today Chet and I cross over to Mexico.

The first dry stream bed turns into a box canyon. Backing out, we head up the steep slopes instead, into the high Mexican desert. The variety of life on the arid, inhospitable hills is astonishing. While the desert is quiet, the struggle for survival is evident in the defense systems of the desert flora.

Lechuguilla (Mexican dagger) plants radiate spiny shoots. Eating them is out of the question; even trampling them is tricky and usually painful. The segmented cholla cactus is among the nastiest, and even a seemingly thornless variety of prickly pear cactus leaves fuzzy red hairs in unprotected skin, first stinging and then itching.

It's spring and the desert is in bloom. The thorny mesquite trees are thick with beige blossoms, and yuccas send up short-lived plumes of white flowers. By the time we return to the Burro Bluffs camp my once-tan trousers are red with bloodstains below the knees. Members of the group would report squeezing thorns out of their skin several weeks after returning home.

Sleeping in the shadow of the cliffs, we are awakened by a brief shower. But the air is so dry that very little rain actually makes it to the ground, and it's over before anything gets wet.

In the morning we rush through breakfast clean-up to climb a trail that ends at the edge of Burro Bluffs' precipice. From this giddy, 1,000-foot lookout, the view into Mexico shows that the rugged hills we climbed last night are just foothills of even higher, more desolate mountains in the distance.

Jay and Jeremy experiment with gravity, tossing rocks into the canyon and timing their fall. The experiment reaches a climax when they trundle a boulder the size of a twin bed over the edge. This pair, the Sierra Club's worst nightmare, will finish the trip with the blood of a Rio Grande carp on their hands.

"Well, they say males are still little boys, even when they're grown up," observes Betty Wilcox. It is a tribute to her restraint that this is her only public editorial on Jay's antics all week.

There is much talk of snakes -- the trans-Pecos copperhead in the cane bushes, the black-tailed rattler in the rocks, 30 species in all. A great deal of attention is spent looking for snakes during forays into the desert (and not just out of idle curiosity.) But the week yields just one sighting -- a nonpoisonous water snake in a pool up one of the side canyons.

Putting on his pants one afternoon, Jay discovers a small scorpion on his trouser leg. After posing for photographs the little arachnid goes into an aspirin bottle as a souvenir.

Other wildlife sightings include big carp wallowing in the shallows with their scaly backs out of the water, ubiquitous buzzards wheeling on desert thermals, a half-dozen wild turkeys flushed from one campsite, and javelinas -- a type of wild pig -- scurrying up a hill near the end of the trip.

In addition, Charlie reports seeing a leaping muskrat, but "El Viejo" is considered a reliable source only in matters of geology. (Kangaroo rats are listed in the guidebooks.)

We begin our next-to-last day with a hike up San Francisco Canyon. Here we find the carcass of a ring-tailed coati-mundi under a rock, and sheep are spotted on the last day. We do not encounter the wild burros said to live along the river.

As the trip winds down, so do the canyon walls. At the last campside we meet up with the other half of our group and camp at Middle Watering, a large, grassy floodplain in Mexico.

The afternoon is blistering hot, but the canyon rim is low. I climb, hands and feet, up onto the gently rolling desert while my companions swim to the United States to sun themselves on a rock. Their resemblance to marine mammals, both in behavior and shape, does not go unnoticed.

On the last morning of our trip, we wake to a drastic change: In less than 24 hours, the temperature has dropped from nearly 100 degrees to the low forties, and the wind is brisk from the north.

In our final nine miles we encounter one shallow rapid -- nothing threatening, but most of us lift past it rather than risk getting splashed in the raw, overcast weather.

We reach the take-out at Dryden Crossing, still a long way from civilization. There's an open shelter with tables and a fence to protect parked vehicles. Across the river we watch a vaquero -- a Mexican cowboy -- riding the ridge on horseback.

After a couple of hours of sorting and packing, the boats are stacked three and four high on roof racks and trailers. Finally the doors squeeze shut.

Alas, the lead van is unable to climb the first hill out of the parking lot. Everyone piles out to push the wheezy old Dodge to the top, and we're finally on our way to Odessa.

For most of us, it's back to the mud of a New England spring. Our intrepid guides will remain in Odessa, awaiting the arrival of the next party they've enticed to see the lower canyons of the Rio Grande.

WAYS & MEANS GETTING THERE: Midland-Odessa Airport, 220 miles northeast of Big Bend, is as close as you can get to the Rio Grande's canyons on a major airline. American Airlines' current round-trip fare from Washington is $398, with restrictions. But the problem is getting your canoe there. This logistical impasse kept my companions and me at a safe distance for more than 10 years despite several sessions spent salivating over maps and guides. We finally decided to quit carping and pay for the convenience of going with an outfitter. Even the cheapskates among us now defend the extravagance. WHEN TO GO: "Any time but spring break -- stay the heck away from that," according to a Big Bend park ranger. The Park Service limits the number of people allowed in each zone along the river, with space allotted on a first-come basis. College spring-break weeks are the only times people get turned away, he said.

Some outfitters schedule around the full moon since it lights up the desert night.

Locals say the best time is late February or early March, when it's warm but not hot and the desert is likely to be in bloom. June, July and August are too hot; December through early February are too cold for comfort by Texas standards. OUTFITTERS: Sunrise County Canoe Expeditions (Cathance Lake, Grove Post Office, Maine 04638, 207-454-7708) arranged our airline travel, accommodations and ground transportation as part of a package. The company's 1991 prices for the week-long trip are $895 single and $1,649 double from Odessa, with discounts for early payment. Discount airfares to Odessa are also available through SCCE.

Local outfitters offering fully provisioned and guided trips through the lower canyons include:

Big Bend River Tours, Box 317, Lajitas, Tex. 79852, 915-424-3219. Specializes in guided raft trips, though it will provide guides for people with their own canoes. Raft trips cost $80 per person per day for groups of five to 15. They can pick you up in Midland-Odessa.

Far Flung Adventures, Box 377, Terlingua, Tex. 79852, 915-371-2489. One- to seven-day trips on the Rio Grande are offered. There is a charge of $600 per person for groups of four or more on a seven-day trip on the lower canyon. Lower canyon trips meet in Marathon.

Local outfitters also run raft trips ranging from a half-day to four days in the Rio Grande's upper canyons, most of which are within the national park.

For anyone resourceful enough to plan his own trip, Mike and Sharon Scott (P.O. Box 405, Sanderson, Tex. 79848, 915-345-2268) run a shuttle service that ferries cars from La Linda to a secure parking area at the Dryden Crossing takeout spot. They also rent canoes for $160 a week. INFORMATION: "The Lower Canyons of the Rio Grande" is a comprehensive guidebook available for $14.95 from the author, Louis F. Aulbach, 3002 Helberg Rd., Houston, Tex. 77092. A excellent and inexpensive set of river guides (maps and text) for all the canyons is published by Big Bend Natural History Association in cooperation with the National Park Service. The river guides, other information, and good advice are available from the Superintendent, Big Bend National Park, Tex. 79834, 915-477-2251.

Stephen B. Collins, originally from Arlington, is a writer and photographer living in Oakland, Maine.