We slice a sparkling trail along the Guapore River, voyaging upstream with Bolivian swampland to our right and Brazilian rain forest to our left. Birds glide overhead, flying between habitats, between countries. Killing the outboard motor, we focus our binoculars and trade identifications. The silenced boat bobs midstream to the rhythm of exotic poetry: spangled cotinga, smooth-billed ani, lesser kiskadee, black-crowned tityra.

It is late afternoon in Noel Kempff Mercado National Park. To get here, we flew by Cessna for three hours out of the city of Santa Cruz, landing at the private airstrip of Flor de Oro Lodge. Within recent memory, this remote area was a wild frontier in the worst sense. It was here, in 1986, that Bolivia’s foremost conservationist, Noel Kempff, was gunned down by drug traffickers.

The park that bears his name has since flourished as a living memorial. Its diverse habitats support an astonishing array of life, including around 700 bird species. Biodiversity on this scale is beyond comprehension. Every time we leave the lodge, we must take with us a small library of field guides to verify the day’s sightings.

With the dusk sky aflame, we slip into a glassy lagoon. The vegetation on all sides is darkly tangled, drooping over the water, melding with it in a confusion of reflection and actuality We share the lagoon this evening. With what, exactly, we are not sure.

Just ripples here, ripples there. A soft snort. A teasing splash. Then, as if deliberately surrendered to us, a lingering view of the bulbous forehead and long snout of a river dolphin.

According to our mammal guidebook, it should not be here. We are 100 miles south of its confirmed range. Its presence — and there will be many more in the next five days — reminds us that this area is on the fringe of the known zoological world. We must put less trust in books, and more in our own eyes.

I have been in Bolivia for two weeks. What is the country like? I could give dozens of answers, each entirely different. Here, in the east, more than a hundred miles from the nearest settlement, it is a humid, pungent, uninhabited wilderness. In the south, on the vast salt flats of Salar de Uyuni, it is a stark, dazzling, mirage-fractured wonderland. In the Yungas, the jungle-choked valleys of the Andes, it is a place where you can trek between the sweltering rain forest and snowy mountain passes in a single day. And on the Altiplano, the high Andean plateau where the capital, La Paz, is located, Bolivia is a country that pushes outsiders to their physical limits.

Adaptation doesn’t happen overnight. I wake up breathless and with an aching head: the inevitable consequences of having pitched my tent 12,000 feet above sea level on the pebbled shore of Lake Titicaca, 43 miles from La Paz.

On the far side of the lake, snowy Andean peaks ascend out of a ruffle of delicate cloud. Faced with this spellbinding view, I find it hard to believe that I am still in the tropics. But the rising sun delivers confirmation, piercing the cold air with vicious intensity. I smear my face with sun block, then bolt down a liter of water to stave off soroche — altitude sickness.

After taking down the camp, we stow the tents and our belongings in our transport for the next three days — sea kayaks. I fix myself into the cramped cockpit and launch from the shore. The rocky shallows descend into deep blueness. An Andean gull tips down and glides across my bow.

The water splashing off the paddle blades is needle-sharp on my bare hands. This lake is fed by melting snow. I am determined not to spill from the boat, and keep my knees firmly wedged in the sides for balance. Here, even so close to shore, the lake is about 900 feet deep.

Our starting point, a sheltered bay beside the small town of Copacabana, is lost behind us in the quivering distortions of UV haze. Ahead of us, our destination is gaining clarity. It is possible now to make out the contours of the steep island that the Incas regarded as the birthplace of their civilization: Isla del Sol — Island of the Sun.

To reach the island we must cross the Yampupata Strait. We assemble as a close flotilla — five kayaks and a support boat — before embarking on the crossing. Out here, stirred by the wind, the water is ridged and occasionally white-capped. Waves break over the bow of my kayak.

To our right, the moon is rising above the appropriately named Isla de la Luna. To our left, the bald hills of Peru roll around the western shore. The lake is shared by the two countries; the precise line of the border is a matter of watery conjecture.

Titicaca is the world’s highest navigable lake, though we encounter few other boats: just a couple of small local ferries and a tourist hydrofoil. Disappointingly, we do not see any totora reed boats, the traditional craft that inspired Thor Heyerdahl’s theory of ancient migration between continents. (His Ra II reed boat, which successfully crossed the Atlantic, was constructed by boat builders from Lake Titicaca.)

Reed boats remain in active service around the southern island of Suriqui. Elsewhere on the lake, sailors have switched to wooden vessels, which, unlike those made from reed, do not become waterlogged and rotten after just six months of use.

When we reach the safe shelter of Isla del Sol by late afternoon, the flotilla strings out. After the two-hour crossing, my shoulders and back ache and my hands are so stiff on the paddle that I wonder if I will ever be able to relinquish my tight grasp. But as I turn the kayak toward the sinking sun, with our campsite at Kona Bay now visible, all weariness drains from me. The paddle blades find easy purchase, and the kayak slices gloriously through the water. I leave my companions behind. For the final hour, I am convinced that I could paddle forever.

The Incas knew that nothing, not even the sun, lasts forever. By the time I reach land, it is dusk. Hauling my boat ashore, I am sharply reminded — throbbing temples, heaving lungs — that I have not yet acclimatized to the altitude. I empty the watertight stowage compartments and join the others in setting up the camp. In the waning light, the first few bats of the evening fly with the last few Andean gulls. Later, only the flitting bats remain.

I stand beside the water with the full moon casting my shadow short on the beach. This was how it was in the time of the Incas. Nothing has changed. The night sky, the reflecting lake, and a man standing here with his face red from a day in the sun and his body weary from the crossing from the mainland.

Today the environs of Lake Titicaca are scattered with Inca ruins and the hills are ridged with the terraces they carved. Something else of them, something less physical, lingers on the shores of this lake. In pure, solitary moments it can almost be touched: a sense that they were a people molded by their remarkable environment; and a feeling that even transient visitors to this place will be indelibly impacted by it.

TAKE NOTE

Bolivia is the archetypal “land of contrasts.” Several tour operators provide guided itineraries that embrace the country’s diverse attractions. Seattle-based Wildland Adventures (tel 800 345 4453, http://www.wildland.com) has a nine-day trip centered on the Noel Kempff Mercado National Park, staying at Flor de Oro Lodge. The same company also offers treks in the Andes and on Lake Titicaca’s Isla del Sol. Responsible Travel (tel 44 1273 600 030, http://www.responsibletravel.com), founded by Dame Anita Roddick of The Body Shop, provides a range of carefully selected sustainable holidays, including a 20-day Bolivian tour that incorporates kayaking on Lake Titicaca, trekking in the Andes, and a visit to Madidi National Park in the Amazon. Explore Bolivia (tel 303 545 5728, http://www.explorebolivia.com) offers a nine-day Andes and Amazon sampler that includes a kayak trip on Lake Titicaca and a motorized canoe expedition in the Amazon basin.