Lac du Cos

Lac du Cos sits at 2,182 metres in the Belledonne massif, within the municipality of Le Haut-Bréda in the Isère department of the French Alps. Covering roughly 20 hectares, it ranks among the larger bodies of water in the Sept Laux (Seven Lakes) group, a cluster often described as one of the densest concentrations of high-altitude lakes anywhere in the French Alps, with more than a dozen named lakes scattered across the same plateau.

Where exactly is the lake?

The lake lies at the head of the Haut-Bréda valley, above the site of the former village of La Ferrière, now part of Le Haut-Bréda commune. Downstream and to the south-west sits Lac Cottepens, part of the same chain. From the shore, walkers get open views toward the Îlettes ridge and the Col de la Vache, while a rocky summit above the water — sometimes referred to locally as Le Toit or the Rocher du Lac du Cos, at 2,835 metres — dominates the skyline.

How do you get there?

The standard approach starts at Fond de France (1,087 m), the point where the D525a road up the Bréda valley from Allevard comes to an end and the trail network begins. From there, the path climbs through the Sept Laux plateau; depending on the exact route chosen, reaching the lake takes several hours and a substantial amount of elevation gain, and the hike is generally rated as demanding rather than easy. A second option approaches from Le Rivier d'Allemont (1,265 m), climbing around 900 metres of ascent in roughly three hours. This stretch of trail also forms part of the long-distance GR738 route.

What is the setting like?

Trails leading up to Lac du Cos cross classic high-alpine terrain, and chamois and ibex are a regular sight along the way. The lake stays frozen for a large part of the year, only opening up as the snow melts, which effectively narrows a sensible visiting window to summer and early autumn.

Can you fish here?

Like the other lakes on the Sept Laux plateau, Lac du Cos holds brook trout (truite fario). Fishing on lakes above 1,000 metres is typically permitted from the first Saturday of June through the second Sunday of October, subject to a minimum catch size (23 cm for most salmonids). Given how long and strenuous the approach is, anglers who make the trip treat a catch here as a reward earned the hard way.

Frequently asked questions

How high up is Lac du Cos? The lake sits at 2,182 metres above sea level, in the Belledonne massif.

What's the best starting point for the hike? Most hikers begin at Fond de France, at the end of the D525a road from Allevard; an alternative approach starts from Le Rivier d'Allemont.

Does the lake freeze over? Yes — it remains ice-covered for much of the year and is only fully accessible in summer.

Are there fish in the lake? Yes, brook trout are present, and fishing follows the seasonal rules that apply to Isère's high-altitude lakes.

What wildlife might you see nearby? Chamois and ibex are commonly spotted on the slopes surrounding the lake, typical of the high reaches of the Belledonne massif.