Lac glaciaire de Bionnassay

Lac glaciaire de Bionnassay is a small proglacial lake fed by the Bionnassay Glacier, sitting in the commune of Saint-Gervais-les-Bains (Haute-Savoie) at the heart of the Mont Blanc massif. The lake forms directly from glacial meltwater and changes shape and size through the season, a common trait of lakes fed straight from an active glacier.

Where exactly is the lake?

The lake lies above the hamlet of Bionnassay (around 1,320 m altitude), along the approach toward Nid d'Aigle and the glacier front. Its outflow runs down the valley as a mountain stream that eventually joins the Bon Nant river.

How did this glacial lake form?

The lake sits right at the foot of the glacier's tongue, in the zone where the ice is melting and retreating. Rocks and boulders carried by the moving ice occasionally break loose from the glacier front and fall into the water, so the immediate surroundings call for caution. The lower part of the glacier is often covered in rock debris, which changes its appearance from one season to the next.

What is the Bionnassay Glacier itself?

The Bionnassay Glacier covers roughly 430 hectares and stretches close to 5 km, making it the smallest of the major glaciers on the French side of Mont Blanc. Its tongue ends between two prominent lateral moraines, visible from several viewpoints reachable from Saint-Gervais Mont-Blanc.

How do you get there?

The easiest way in is the Tramway du Mont-Blanc (TMB), the highest railway line in France. It links the Saint-Gervais–Le Fayet station (580 m) with the Nid d'Aigle station (2,372 m), from where hiking trails continue toward the glacier. A short viewpoint loop takes about an hour round trip and is rated easy. A longer descent along the moraine, closer to the glacier front, takes roughly an hour and a half and is more demanding, with sections secured by ladders. Hikers can also connect to the GR5 trail toward Bellevue, and the route includes a suspension footbridge over a glacial stream.

What should you know before setting off?

This is high-mountain terrain right at the front of an active glacier, so conditions and trail access shift from season to season, and falling rock near the ice front is a real possibility. The route toward Nid d'Aigle runs seasonally; it is worth checking the tramway's current operating dates and trail conditions with the Saint-Gervais tourist office before heading up.

Frequently asked questions

How do you reach the Bionnassay glacier and its lake? The simplest way is the Tramway du Mont-Blanc from Saint-Gervais–Le Fayet station to Nid d'Aigle, then on foot along the viewpoint trails.

Is it safe to get close to the lake? The area right at the glacier front calls for caution, since rocks can occasionally fall from the moving ice. Stick to marked trails and viewpoints.

How long does the hike to the glacier viewpoint take? The short, easy viewpoint loop takes about an hour round trip; the longer descent nearer the glacier front takes around ninety minutes and requires better fitness.

Is the Bionnassay Glacier a large one? It covers about 430 hectares and runs almost 5 km long — the smallest of the great glaciers on the French side of Mont Blanc, though its wild setting still makes a strong impression.

Where does the lake's water go? It drains through a mountain stream down the valley that joins the Bon Nant river.