Swimming in Lake Annecy: Water Temperature, Spots and Open-Water Rules

There are two ways into this lake: bathing — ten metres, a plunge, a towel — and swimming. The second has its codes, its season, its kit, and an exceptional playground: water among the clearest in Europe, legible shores, and a legendary event that crosses the lake every summer. Here is open water in Annecy, taken seriously.

The temperature, month by month — and at depth

The surface trails the summer: cool in June, 22-24 °C at the height of summer, still kind in early September — the lake runs weeks behind the air. That’s why the late-September triathlon is still swimmable, and why June always disappoints the impatient.

But the number that matters to the open-water swimmer is elsewhere: below about fifteen metres, the water stays between 6 and 8 °C all year. Moving offshore, you swim above a cold reservoir — and the thermocline can be crossed just a few metres down. A crawl offshore is not an extended bathe: it’s another discipline.

Where to actually swim

• Along the shores, early in the morning. Before the boats and the wind, the water is a mirror: this is the local swimmers’ slot. The beaches serve as entry and exit points.
• The marked bathing zones for easy sessions: the lake is busy with boats, and the yellow buoys are not decorative.
• The bay at the far end for the warmest water — depth explains everything: shallow, it warms fast and cools late.

Golden rule outside marked zones: a visible swim buoy (the orange float tied at the waist). It doesn’t carry you; it makes you visible to boats — the most important piece of kit after the suit.

The lake crossing: the fixture

Every summer, in mid-August, hundreds of swimmers set off on the lake swim crossing — Annecy’s great popular open-water event, backed by the tourist office and relayed in our calendar each year. It’s the perfect goal for a summer of training: within reach of a regular swimmer, unforgettable at first light.

The wetsuit, without dogma

• July-August: optional for most, over sensible distances.
• June and September-October: recommended beyond a quarter of an hour — the body cools faster than a swimmer feels it.
• The rest of the year: thick-neoprene territory, for seasoned swimmers. The lake almost never freezes, but 8 °C doesn’t need ice to be serious.

What we wouldn’t do

• Cross the lake outside the organised event. Two kilometres of navigable water, alone and invisible: no. The crossing exists for that.
• Swim offshore without a visibility buoy. Boats do not see you — being visible is your job.
• Trust the first few minutes’ sensation. Water that feels “fine” on entry chills a resting body fast; at the first shiver, get out.
• Dive offshore after a heatwave without acclimatising. The thermocline’s thermal shock surprises even the regulars.

The lake rewards swimmers the way it rewards early risers: with an hour when it belongs entirely to them. A crawl at 7.30 a.m. in 22-degree water, the mist lifting, the Semnoz in silhouette — it’s the finest swimming pool in the Alps, and it’s free.