How Deep Is Lake Annecy? Down to 82 Metres

The answer comes in two numbers: Lake Annecy drops to about 82 metres at its deepest, with an average depth of 41.5 metres. But those two figures hide what matters — where that deepest point actually is, why one half of the lake is nearly twice as shallow as the other, and what all of it changes for the water itself.

82 metres at the deepest — and where it hides

Contrary to what you might picture, the deepest point is not in the middle of the lake. It is a hole: the Boubioz, a funnel that opens off the north-west shore, about 250 metres out, not far from the Marquisats and Albigny beaches. It starts around 20 to 25 metres down and plunges past 80.

This is more than a quirk of the lakebed. At the bottom of the Boubioz rises an underwater spring, fed by rainwater seeping through the limestone of the Semnoz — some of what fills the lake comes from no visible river at all. The whole regime of the lake plays out down there, in a chasm invisible from the surface.

Two lakes in one: the Grand Lac and the Petit Lac

The second thing “82 metres” doesn’t tell you is that it only applies to half the lake.

The lake narrows at Duingt, between the château on its peninsula and the Roc de Chère, into a channel barely 800 metres wide. That sill divides two basins. To the north, facing Annecy, the Grand Lac — the one that holds the 82 metres. To the south, from Talloires to Doussard, the Petit Lac, “the far end of the lake,” is markedly shallower.

That difference is not trivia. It is why the Bay of Doussard, in the south, is always the warmest water in the lake: shallow, it has no great cold reservoir beneath it. A useful detail when choosing where to swim.

What depth really changes

Depth is the hidden variable behind the lake’s three defining traits.

• Temperature. Below about fifteen metres, the water stays between 6 and 8 °C all year. That cold, heavy reserve is what depth makes possible — and it’s what catches you out when you dive in offshore.
• Freezing. Such a mass of water takes weeks of extreme cold to lose its heat: that is why the lake has not frozen over completely since 1891.
• Clarity. Deep, cold, slowly renewed water is water poor in algae — one of the reasons it is so transparent.

The lake in five figures

• Maximum depth: ≈ 82 m (at the Boubioz hole)
• Average depth: 41.5 m
• Length: ≈ 14.5 km
• Area: 26.5 km²
• Volume: ≈ 1.1 billion m³

What we wouldn’t do

• Assume the middle of the lake is deepest. The low point is off-centre, against the north-west shore.
• Assume the whole lake is deep. The Petit Lac, in the south, is far flatter — and far warmer in summer.
• Treat the Boubioz as a swimming spot. It is a technical dive site: cold water, serious depth, for trained and supervised divers only.

Lake Annecy fits into a single number — 82 metres — but everything that gives it character lies in what that number sets off: cold water at the bottom, rarely locked by ice, and clear for a long time to come.