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We’re sipping chestnut kir on a terrace overlooking the Tarn River in southern France when we hear excited voices from the table beside us: “Regards! C’est un castor!” Below us, a beaver the length of my leg is languidly swimming upstream. We don’t need our binoculars because the Tarn is so clean that almost every fish, frog, pebble and ribbon of weed can be seen with the naked eye, magnified by the clarity of the water. This meandering, jade-green river – which winds from its source in the Cévennes national park to Moissac, just north of Toulouse – is home to trout, perch, carp, otters, frogs, toads, kingfishers and herons. We add “beavers” to our list.

Above us, huge vultures have been drifting all day, cruising the thermals in groups of nine or 10. And when our eyes haven’t been on the river or the sky, they have been welded to the many orchids on the bank: including monkey, bee, military, butterfly, pyramidal and fragrant orchids. Later, we discover that 30 varieties have been recorded in this orchid hotspot.

Enticed by the cooler microclimate provided by the double whammy of a river and a deep gorge, we’re walking a five-day section of one of France’s newest long-distance hiking routes (April, May, early June and late September are among the best times to tackle it). The 300km GR736 officially opened in 2023 and runs from the Tarn’s source to the city of Albi. Three days of the route run directly through the Gorges du Tarn, Europe’s longest and most dramatic canyon, a 33-mile (53km) limestone gully of rock formations and towering cliffs that often rise 500 metres above the river. The gorge is also home to more than 3,000 vultures, as well as cuckoos, nightingales, red-billed choughs and owls.

Besides exceptional wildlife, a succession of medieval towns, abandoned hamlets, deserted churches, ruined castles, crumbling terraces and jaw-dropping architectural oddities are dotted along the gorge. Most must be reached on foot, via “balcon” paths often dizzyingly (albeit safely) whittled from the gorge itself.

Our walk begins in Le Pont-de-Montvert, a bustling historic town amid the expansive heathery uplands of Mont Lozère, from where the Tarn springs. For two days we traverse an unpeopled, wind-blown wilderness of menhirs, boulders, broom and heather, before descending to wildflower meadows and forests of beech and pine. We walk about 10 to 15 miles a day, unimpeded by bulky backpacks as our luggage is transported in a minivan that arrives promptly at 9am every morning.

It’s only as we descend into the gorge on day three that we begin to encounter a myriad of human-made curiosities. The first is Castelbouc, a semi-troglodytic hamlet of narrow, cobbled streets, watched over by the remnants of a castle balanced on a vertiginous spur.

From here, we briefly cross the gorge to Sainte-Enimie, one of France’s celebrated “plus beaux villages” (its steep cobbled streets are lined with honey-coloured houses hung with pink roses), which is thriving thanks to the road that winds along the gorge’s right bank. Then it’s back to the left bank with its single-file footpath and cooling canopy of chestnut trees. We stop for the night in the restored village of Saint-Chély-du-Tarn, which has an original village bread oven, waterwheel, 12th-century church and miniature chapel carved into the rock. We dine at Auberge de la Cascade (the only place to eat in the village), feasting on Tarn-caught trout and ice-cream made from the local châtaigne, a small sweet chestnut and essential ingredient in the region’s most delicious aperitif, kir à la châtaigne – white wine with a dash of chestnut liqueur.

The following day’s gorge walk feels weirdly Jurassic, thanks to the sulphurous-yellow rocks, thickly mossed tree trunks and enormous ferns. At the hamlet of Hauterives, we spot a rigged cable-and-basket for hoisting goods over the river: someone appears to be living in this isolated spot. A couple of miles of ascending and descending brings us to the waterside town of La Malène, where we relax in a flat-bottomed boat while a professional batelier (boatman), Clément, punts us four miles downriver, pointing out the beaver families that inhabit every half-mile of riverbank, and four varieties of resident vulture (griffon, black, bearded and Egyptian), whose young will fledge throughout the summer.

Clément puts us ashore at Cirque des Baumes, where we scramble up the almost sheer bank using a series of ropes – an exhilarating experience. Here, we encounter yet another hauntingly abandoned village where miniature houses perch – precariously and mind-bogglingly – on limestone plinths. Signs of life (washing on a line) suggest that someone also lives in this ghost village – without electricity, gas or mains water.

We stroll on, past miles of neglected retaining walls, which once held terraced orchards of cherry and chestnut trees; now, the place is home to deer, boar, pine martens and muskrats. After a cooling dip in the river, we walk to Les Vignes and our charming hotel, Le Parisien – pink-and-white striped walls, vintage framed photographs, antique brass beds. The chef, Amélie, tells us that she hopes the new GR736 will bring more people to the area, providing new life for these dying villages.

On our final day, the gorge slowly unfolds and flattens, opening out into a verdant valley of twisted triffid-like trees draped in luxurious moss, and glistening banks of black volcanic stone spill across our path. We follow the river to its confluence with the Jonte and spend our last evening in Peyreleau (designated “une petite cité de caractère”), strolling ancient cobbled streets and exploring medieval churches and towers with far-flung views. From the terrace of Hôtel Doussière, we watch swallows and swifts skimming the river below, and agree that Robert Louis Stevenson was right when he wrote: “If the garden of Eden exists, it’s in the valley of the Tarn …”

Annabel travelled independently. On Foot Holidays offers a week-long, self-guided trip with baggage-carrying service from £1,245pp. More information at gr-infos.com

Annabel is the author of The Walking Cure, published by Bloomsbury Tonic (£10.99). To support the Guardian buy a copy at guardianbookshop.com. Delivery charges may apply