Serre-Ponçon Swim Challenge

Before dawn breaks over the peaks of the southern French Alps, small green safety lights glow across the dark surface of the Lac de Serre-Ponçon. The Serre-Ponçon Swim Challenge is an open water swimming marathon held in the pristine waters of this immense alpine reservoir at Savines-le-Lac, in the Provence-Alpes-Côte d'Azur region. Organized in July by the Association Serre Ponçon Swim Challenge, the race takes endurance athletes along the flooded valleys of the high mountains.

The Serre-Ponçon Swim Challenge: 12 and 20 kilometres

Swimmers can test their endurance across two ultra-distance courses of 12 and 20 kilometres, contested in both Solo and Team Challenge categories. Every solo swimmer must navigate alongside a dedicated support kayaker for the entire journey, while team entrants alternate between paddling and swimming roles throughout the course. To make logistics seamless, the organizer supplies each team with a kayak, paddle, buoyancy aid, radio, whistle, and a Restube safety buoy. Each support paddler must submit a signed certificate proving their kayaking competence, ensuring reliable guidance and hydration support stroke after stroke across the vast expanse of open water. Safety remains tightly coordinated from the moment athletes step into the lake.

To meet the race time cut-offs, participants must be capable of maintaining an average pace of at least two kilometres per hour. Fins, masks, snorkels, and other swimming aids are permitted unless deemed dangerous by the organizers, opening the marathon format to fin-swimmers and traditionalists alike. The alpine summer typically brings pleasant swimming conditions, with expected water temperatures sitting comfortably between 18°C (64 °F) and 21°C (70 °F). Wetsuits are mandatory only if conditions cool significantly and the water dips below 16°C (61 °F). In past editions, swimmers encountered pristine mornings where the lake lay as smooth as polished glass beneath warm air. Entrants described the tranquil silence of stroking through the mountain-framed basin as the sun rose, supported by the quiet rhythm of their companion kayakers in an atmosphere of shared camaraderie.

Lac de Serre-Ponçon: Alpine waters of Durance and Ubaye

The race takes its name and character from the Lac de Serre-Ponçon, a massive artificial reservoir created in the late 1950s by damming the Durance River. Construction of the great impoundment wrapped up around 1959 and 1960, flooding the ancient junction where the Ubaye River merged into the Durance. The Ubaye flows down across 83 kilometres from its origins at the Col de Longet in the Cottian Alps along the Italian border, pouring into the lake near La Bréole. Spanning an impressive surface area of 2,800 hectares, the lake extends up to 20 kilometres in total length and winds past approximately 91 kilometres of rugged shoreline. The water quality is systematically monitored each year and achieved a pristine "very good" classification in 2022. For marathon swimmers, this alpine basin provides pure, clear freshwater flanked on both sides by sweeping mountain flanks that shelter the long stretches of open water.

The drowned valley: Savines, Ubaye, and the Chanteloube viaduct

Gliding across the surface means swimming over historic ground. When the reservoir was filled in 1961, the rising waters drowned the ancient village of Savines, forcing residents below the 784-metre contour to leave their homes before the original parish church was dynamited. Further south, the historic village of Ubaye was completely submerged and erased, never to be rebuilt. Modern road traffic spans the wide basin via the Savines Bridge, an imposing 924-metre concrete viaduct built across the dry valley floor before the flooding occurred, supported by hefty square piers more than 16 feet wide. About eight kilometres down the lake lies another striking relic of the lost valley: the Chanteloube viaduct. Because the viaduct's eastern trestle stands at 780 metres—close to the lake's nominal pool elevation—the stone arches re-emerge like a ghost bridge during seasonal drawdowns, while in summer the surrounding sheltered bay becomes a popular haven for swimmers and boating enthusiasts.

Îlot Saint-Michel: the chapel spared by the reservoir

Among the most remarkable sights in the basin is the Îlot Saint-Michel, a tiny rocky outcrop measuring just 90 metres across. Perched on top sits a chapel originally founded in the 12th century at the former hamlet of Les Couches. Paddling and swimming near this isolated islet gives competitors an unforgettable sense of scale, highlighting how the high dam transformed a secluded alpine river bed into an inland sea.

Demoiselles coiffées de Pontis: stone pillars above the shore

Along the shoreline about five kilometres away from the race area, the slopes feature the striking geological curiosities known as the Demoiselles coiffées de Pontis. These atypical rock formations, often referred to as fairy chimneys, are slender pillars of compacted earth each crowned by a large, protective boulder balanced precariously on top. Local folklore has long surrounded these bizarre natural towers with myth, claiming they were young village women who were dancing at a festival and were turned to stone after a petrifying cry echoed from the surrounding mountains. Out in the middle of the lake, swimmers find clean water, quiet alpine air, and miles of uninterrupted swimming beneath the mountain summits.