Crocodiles and iguanas and herons. Oh, really?

AS our launch carried the 10 of us down La Tovara River in Mexico’s Nayarit state, I had one thing on my mind: Would I see a crocodile?

Our eco-tour near the sleepy coastal town of San Blas had promised a lush array of tropical vegetation and exotic turtles, snakes, birds and iguanas, but disappointments on such past excursions had made me and my husband, Richard, skeptical.

After all, we were only 100 miles north of the busy, tourist-filled streets of Puerto Vallarta. Could we really expect to see all the requisite animals on this jungle river tour?

Indeed we could.

No sooner had we cleared the first bend of La Tovara when there, perched on a mossy branch, was a 2-foot-long iguana. Seconds later, we gasped: On the opposite shore was a 3-foot crocodile snoozing on a soggy log.

The next hour was one animal thrill after another as we floated on murky water teeming with sea bass, clams, porgies, mojarras and other fish. All around us were thickets of trees swathed in orchids and vines, with iguanas and exotic birds anchored in them. Crocodiles dozed and waded in the shallow water; shy turtles basked on rocks.

The ecologically rich setting surprised me, but it shouldn’t have. Mexico is one of the most bio-diverse countries in the world. It ranks in the top 10 for its range of reptiles, mammals, amphibians, vascular plants and birds, according to Vivanatura, a nature conservation project linked to the Mexican Conservation Organization. The country has 14 major vegetation zones, and seven of them are in the Puerto Vallarta region, which is on the same latitude as Hawaii.

In this marshy estuary on Matachen Bay, spring water and saltwater merge to nurture a tropical deciduous forest. Our foray might seem placid and scenic to the untrained eye, but thanks to Hugo, our alert boatman, and Jorge, our guide, it was an adventure through an environmental wonderland.

Early on, we spotted a golden-orange butterfly taunting a 2-foot turtle. From a slimy green rock, the turtle leapt into the water to escape the pest and our prying eyes.

Moments later, Hugo pointed to a clutch of lissome great white herons fishing near the bank. As we sneaked up, we caught glimpses of orange and red orchids nestled in the elbows of sloping madrono trees, which also harbored giant termite nests.

The scene was a bizarre, spellbinding confluence of languid Louisiana swamp, Everglades intensity and Venetian grace.

Hundreds of stilt-like trees, which Jorge identified as mangroves, lined the dense marsh. Towering above them were banyan trees with jumbled roots bulging eerily above the water.

Hugo deftly routed us around them as he stood in the rear of the boat and navigated with a long pole. Boatloads of other visitors occasionally slipped past. But the dominant sounds here were not other eco-gawkers but the chortles and squawks of tropical and migrating birds.

La Tovara is on the Pacific flyway, a major north-south path for millions of birds that migrate from the Americas and Canada between October and January.

Happy that many had lingered on to coincide with our late-January tour, we marveled at splendidly colored birds pirouetting in an azure sky, at gray falcons circling above and at screeching green canaries spiraling out of a leafy Mexican buttercup tree. It was a bird-watching bonanza.

Jorge pointed out two kinds of flycatchers: one, yellow-bodied with brown wings; the other, a vermillion flycatcher, was smaller and a brilliant orange-red. Both busily lunged at insects, which were plentiful.

Boat-billed herons flapped and thrashed in the dense foliage. Pelicans swooped high in the sky among white egrets coasting in circular formations. We also glimpsed regal gray-blue herons and bright-feathered kingfishers. My favorite was the green heron. With its ragged-edge wings, choppy crown and beady black eyes, the bird looked as though it came straight off a drawing board at Disney.

As we rounded a bend, the idyll was shattered by Hugo’s cry: “Crocodrilo grande!”

As he eased the boat into a hidden pullout along the banks, we spotted the leathery 8-foot beast sunning itself on the sand. Like a swarm of paparazzi zooming in on Lindsay Lohan, we snapped away at it.

Besides crocodiles, there were other fearsome creatures in this Garden of Eden. Hugo and Jorge kept watch for boa constrictors, which sometimes dangle from tree branches. But to our mingled relief and disappointment, none was apparent. Other inhabitants of Mexico’s tropical deciduous forests -- small, armored armadillos, rust-colored spider monkeys and jaguars -- also eluded us.

And we regretted not seeing more orchids in bloom. Nearly 100 species thrive here, and the blossoms commonly open during the spring-summer rainy season. Still, stubborn red romelias occasionally protruded among spindly white spider lilies, exotic plumeria and birds of paradise.

Beyond the thick mangrove marsh, we pulled into Camalota Spring, a clear pond teeming with wriggling fish. We docked there for a visit to Ejido de la Palma, a crocodile farm run by the Mexican government and local citizens to preserve and propagate the endangered reptile. At La Palma, crocodiles are bred and their eggs hatched in safety. Young crocs then are released into the river.

Chain-link enclosures held one or two jumbo crocs -- some 8 feet long. With armor-like dark-green scales and spikes, they looked fittingly prehistoric. We watched them slither out of their pools, roll in dirt, shake off the excess and lumber to a warm spot for a nap.

They ignored the incessant clicking of my camera until I invaded the personal space of a dozing croc. One eye slowly opened and glared. My heart racing, I backed away from the fence.

A few held open their jaws, revealing formidable teeth. Jorge said the cold-blooded reptiles wouldn’t attack humans unless desperate for food.

A less fearful sight were the hatchlings; about 50 newborns were tangled around one another by a wading pool. Another enclosure held 50 slightly older ones writhing playfully.

A spacious enclosure holding a dozen raccoons was our final stop. The gray animals, 18 inches tall with black-and-white ringed faces, mournful dark eyes and striped tails, paced and raced around their pens.

Two standouts were coatis, reddish brown, with longer, cone-shaped snouts and erect, ringed tails. They are indigenous to local jungles and forests. As Jorge cautioned us to stand back, the sharp claw of a curious coati lunged through the fence and tore my plastic bag, which contained a snack.

Unlike many eco-tours I have taken, this one changed my perceptions of bustling Puerto Vallarta. For me, “The Night of the Iguana” had become “The Day of the Crocodiles.”

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A tropical trip

GETTING THERE:

From LAX, Aero California, Alaska and Aeromexico fly nonstop to Puerto Vallarta. Aviacsa, American, America West, Mexicana and Aeromexico have connecting flights. Restricted round-trip fares begin at $164. San Blas is about 100 miles north of Puerto Vallarta.

TELEPHONES:

To call the numbers below from the U.S., dial 011 (the international dialing code), 52 (country code for Mexico) and the local number.

ECO-TOURS:

The following tours can be booked through hotels and travel agencies. Use of insect repellent is advised.

San Blas-La Tovara Tour, 237 Transportes de la Bahia, Juarez Colonia Central, Jalisco, Mexico; 322-222-8370. Tours 7:30 a.m.-6 p.m. Tuesdays and Thursdays. They can be booked at hotels and travel agencies. $75 per person.

Transportes Turisticos Flamingos (TTF), 74 Carretera Tepic, Vallarta Mezcales, Nayarit, Mexico; 322-222-1922, www.ttf.com.mx. Tours 8 a.m.-7 p.m. Tuesdays and Thursdays. $100 per person.

TO LEARN MORE:

Mexico Tourism Board, (800) 446-3942 (for brochures) or (310) 282-9112, www.visitmexico.com.

-- Irene Woodbury