The Usumacinta River, a never-never land of steamy jungle, roaring rapids, mysterious ruins and unusual wildlife, traces the remote border between Mexico and Guatemala, then plunges through steep-sided limestone canyons, ultimately crossing Mexico's coastal plain to the Gulf of Mexico. Along its shores, scattered settlements carve raw scars in an otherwise pristine wilderness.

This is a place where monkeys swim and growl at the moon like angry watchdogs. It is a place where beetles built like Volkswagens glow in the dark when tickled, and where the fer-de-lance, deadliest snake in the Americas, vies for supremacy with the jaguar, this hemisphere's largest cat. It is also a place where hundred-foot waterfalls burst from travertine clefts, where ferns grow tall as houses, and where trailing lianas, strangler figs and flowering orchids and bromeliads festoon giant ceiba trees 120 feet above the forest floor.

If there was ever an area ripe for international park status, this is it. But the future is not bright.

Once a super-waterway of the Maya, the Indians whose civilization mysteriously declined a thousand years ago, the Usumacinta today is threatened by plans for a series of hydroelectric dams which, if completed, would wipe out up to 500 square miles of tropical forest, force untold numbers of people to move and flood dozens of Mayan temples and palaces -- many of which have not yet been explored by archeologists. And although Mexican President Miguel de la Madrid has pledged that any dam built along the border would not damage any archeological sites, ecologists argue that even the construction of roads in the area would be harmful.

Feasibility studies are expected to take at least five years. But whether or not the dams are built, the jungle -- one of the largest tropical rain forests in North America -- may already be in trouble. Homesteaders have been quick to use roads built by surveyors, oil explorers and timber companies. And the practice of milpa -- the traditional slash-and-burn method of settlement, whereby millions of people subsist in the tropics -- has increased, further threatening the forest.

Last March I visited more than a dozen Mayan ruins, most nearby or within a day's hike of the river. Beginning in Belize City, Belize, our group of 14 covered more than 300 miles in three weeks, two-thirds of the distance by water, using locally built flat-bottom river boats and, later, rubber rafts. Frequently camping out along the way, we traced the ancient Mayan trade route linking Tikal, Guatemala, and Palenque, Mexico. In between, shrouded in jungle, are hundreds of other ruins, many of them reachable only by water. Among them are Yaxchila'n and Piedras Negras, two important sites that would be flooded under the hydro power plan.

Early archeologists dynamited many Mayan ruins in their race to collect antiquities; looters have destroyed many others. As a result, some sites look like war zones, and much important historical evidence has been lost. Nor has time been kind. At many sites the jungle has had the last word: Tangled roots have forced arches to collapse and walls to fail; elsewhere, orange lichen has obliterated hieroglyphic detail on once brilliant frescos. Even so, tantalizing hints of a once vibrant, now lost culture capture the imagination at every turn.

It was fitting we should stop at Tikal early in our trip because, although it is not on the Usumacinta, its influence once reached far up and down the river. The park covers 222 square miles. At its core, about 3,000 Mayan buildings have been mapped -- of which only 2 percent have been excavated.

At Tikal, from the top of one pyramid, we gazed for 40 miles in every direction over what is now sparsely settled forest. It is difficult to believe this area was once a vast community of paved roads, plazas, causeways and reservoirs, temples and palaces, supporting almost a million people.

Tikal means "Place of Voices" in Mayan. One evening, as we watched the full moon rise over its great courtyard, one of our party -- a student of Native North American shamanism -- chanted Sioux greetings to the Mayan gods, and burned copal, a local incense. We could easily visualize Mayan priests making similar use of the echoes here to intimidate their subjects.

We met our first boat crew 75 miles southwest of Tikal, at Sayaxche' ("Cedar Fork"), a small Guatemalan town on the Rio de la Pasio'n, a Usumacinta tributary. With the help of a hand-held searchlight, our 45-foot flat-bottomed boat motored slowly through a narrow slough to Petex Petun, a murky, cigar-shaped lagoon five miles long. Twin pinpoint crocodile eyes winked at us from the shallows; bats and large, fluttering moths dove in and out of the light; an occasional splash would signal the unseen movement of an iguana.

This was our first foray into "real" jungle. Having been warned about tarantulas, snakes, scorpions, botflies and other unpleasant aspects of the tropics, we were all somewhat apprehensive. Our fears were largely dispelled, however, when we arrived at a luxurious (by jungle standards) camp situated high on a bluff amid tall trees, overlooking the water. Our two-person tents were set on platforms shaded by thatch roofs, and the camp offered electricity (until 10 p.m.), flush toilets, showers and an outdoor dining area.

We stayed two nights, making daytime forays to nearby ruins, swimming and doing our laundry in the lake. It was hot during the day, but we needed blankets at night, and the unusually cool temperatures kept the ubiquitous mosquitos at bay. Later on, as the weather turned hotter and more humid, we were to remember this shorted-lived luxury with nostalgia.

Only the month before, Guatemalan guerrillas had visited Petex Petun. Since then they seem to have made themselves generally scarce, perhaps because the army is very much in evidence now, especially at roadblocks throughout the countryside. In any case, we saw no guerrillas anywhere.

Instead, we saw settlers scratching out new milpas along the river; sometimes the smoke from their fires hung heavy in the humid air. Women in red and orange dresses pounded clothes clean on river rocks, while children splashed like otters in the shallows nearby. We saw semi-wild pigs scrounging for scraps in the shadow of thatch-roofed huts; razor-hipped cows and horses were grazing on the slopes. The men chopped at the jungle with lethal-looking machetes, or fished from handhewn dugouts.

Occasionally we would see travelers scouting along the shore. They carried small bundles and appeared to be searching for likely river crossings. On the Mexican side of the river we saw these voyagers' abandoned rafts, narrow logs lashed with jungle vines -- mute testimony that some sort of a human migration northward is ongoing.

A birdwatcher's paradise, this. One ornithologist recently counted 200 different birds along the river in 10 days, almost half of all species known to live or visit here. Even I, a birdwatching illiterate, tallied 75, many classified as rare or endangered. My list included herons, egrets, ospreys, parrots, scarlet macaws, kingfishers, and birds with names like anhinga, guan, jabiru, tinamou and motmot. We also saw warblers, flycatchers, vireos and an indigo bunting -- all North American songbirds that winter in Central America. Our songbirds are, in fact, not really "ours" at all -- most of them spend over half their lives in the tropics.

The closed, dense forest teems with unseen life. We knew jaguars were there, because we saw their droppings on the trail. Once, we saw the delicate track of a tiny brocket deer. Other animals -- tapir, coatimundi, peccari, paca, kinkajou and sloth -- were more elusive. Howler and spider monkeys didn't need to hide -- they just stayed high in the canopy and insulted us vocally, or by throwing down fireplace-sized chunks of wood. Turtles sunned themselves on half-submerged logs, and the tracks of iguanas and small rodents criss-crossed our sandy campsites. One morning there were cat prints a few feet from my tent -- a margay or an ocelot had come to drink at dawn.

Our open longboats -- one for passengers, one for supplies -- were made in Sayaxche of locally abundant mahogany, then painted white. Each had a thatched roof shading its deck, under which we took refuge from the sun. At noon the boats would nose onto the shore. Then we would don bathing suits and cool off in slow eddies near the bank. Soon a picnic of salami, cheese, crackers, sliced carrots, cucumbers and salsa would appear as if by magic, laid out on a seatplank taken from one of the boats.

One of these stops was at Planchon de las Figuras ("Beach of the Figures"), where hundreds of years ago some early Maya had scratched graffiti. Their casual drawings -- an alligator, a temple, circles and swirls -- were so like our own beach doodles that it was easy to think they may have visited this beach on a hot summer's day like this one, with a picnic similar to ours.

With temperatures consistently in the upper 90s, keeping meat, fruit and vegetables from spoiling was a challenge. But thanks to the ingenuity of our crews -- the Guatemala supply boat had a gasoline-powered icebox; the rafts carried blocks of ice in large coolers -- we ate well: local fish, chicken grilled over coals, pork chops, pasta, beans, salads, melons and other fresh fruit, even a birthday cake. On the river we carried a water filter, and pumped fresh water daily into large thermos containers. Otherwise we drank beer, soft drinks and powdered fruit punches.

The river was wide and sluggish until midway in the trip when we reached Corozal, a small army outpost where we checked through Mexican customs. Here we changed guides, and transferred to 16-foot rubber rafts powered solely by oars.

Within a half-hour we came upon the first indication that Yaxchila'n ("Green Stones") was near: a tumble of rocks at water's edge, once a river marker for Mayan travelers. High on the hills behind, shrouded in forest, is Yaxchila'n, a site discovered in the late 19th century: two square miles of temples and palaces, of which only one-thirtieth has been excavated. The latter are well-maintained and watched around the clock by diligent guards.

If the dams are built, only a few buildings on the highest elevations here will escape flooding.

Mexican archeologists have been working at Yaxchila'n off and on since 1972; the restoration of certain major buildings and courtyards rivals similar work at Tikal and Palenque. The ruins date from the Maya's Classic era, from about 300 to 900 A.D. There are fine carved doorway lintels here, and remnants of original red and aqua frescos. A headless, larger-than-life figure seated in lotus position faces the river from the doorway of "Building 33" (named for the order in which it was excavated). Modern Lacandon Indians, direct descendants of the Mayans, believe that the world will end if the head, lying nearby, is set back on the torso.

There is a stone labyrinth at Yaxchila'n: dark, twisting corridors and chambers where the Mayan elite may have practiced bloodletting to induce visions. One day, four of us measured and mapped these halls by flashlight, brushing cobwebs from our faces and occasionally dodging small, disgruntled bats.

By Day 17, when we arrived at Piedras Negras ("Black Stones"), we were seasoned river travelers -- quick to unload the rafts and set up tents and cooksite at day's end, handy with machetes on the trail. In the last few days, the Usumacinta had grown increasingly deep and swift, the hillsides steeper.

Now reachable only from the river, Piedra Negras was excavated in the 1930s by archeologists from the University of Pennsylvania. In 1960 Mayan scholar Tatiana Proskouriakoff deciphered some of the Piedras Negras hieroglyphs -- a major breakthrough for students of Mayan writing, who ever since have been scrambling to reinvestigate other hieroglyphic source material. A derelict tractor from that time still stands guard, settling quietly on rusted wheel-rims in the underbrush.

Here the jungle has closed in; the claustrophobic milieu made the presence of Mayan spirits almost palpable.

It was unbearably hot, and we spent one afternoon lolling about in the shallows, dreaming of cold beers and ice cream, while thousands of blue-and-white butterflies danced on the beach.

Relief was soon at hand. Early the next day a sudden bend in the river revealed Budsil Ha ("Smoking Water"), a spectacular waterfall near one of the proposed dam sites. Here we clambered into caves carved in the travertine rocks and dove through cascades into foamy pools. Soon after, we entered a series of limestone canyons. Almost imperceptibly, our rafts picked up speed. The oarsmen, one to each boat, were suddenly all business, focused on the task of negotiating jutting rocks and whirling eddies.

The Usumacinta's white water is rated a 3. This means its rapids are to be respected, but are not so formidable as, say, those of the Grand Canyon. Even so, we were sometimes whipped around unexpectedly by whirlpools, or flung against a rocky face off of which our pontoons fortunately bounced unscathed. Here and there small trees trailing vines clung precariously to perpendicular cliffs. A pair of eagles soared two miles high above us. The ancients called these canyons Xibalba, entrance to the Underworld.

The Usumacinta, now entirely within Mexico's borders, soon burst into the coastal plain and slowed. It would meander another 100 miles before reaching the sea. But for our group, the river adventure was over. Four hours and many aching muscles later, having helped to unload and dismantle the boats, we were in Palenque, enjoying our first hot showers in three weeks, shaking the sand out of shoes and sleeping bags, and celebrating our return to civilization over a gourmet dinner, with a marimba band challenging our prowess on the dance floor.

One way or another, it's just a matter of time before much of what we saw will cease to exist.

Janet Trowbridge Bohlen writes about wildlife and the outdoors.