Crossing Switzerlands 100 biggest lakes

What’s that all about?

So there we are, somehow I told too many people I was going to cross the 100 largest lakes in Switzerland. Why you ask? Well I think there are many good reasons and I hope to share some of them on this website. Beyond it being a great excuse to go out there visit my own country with some friends, I am also raising money for charity (trying to figure out the details now).

Furthermore, discussing this project with friends uncovered many interesting questions that I’ll attempt to answer throughout this adventure. But I would like to start by presenting the rules of this project.

The rules

1. Swimming with a buoy is mandatory.
2. Someone must accompany me on every crossing.¹
3. Using a wetsuit is allowed but no other swimming helping devices are (no fins, snorkel, underwater jets)
4. Dry land must be touched on both sides.
5. Original trajectory² may be diverted from for different reasons³ but the swimming distance must be at least as long as the crossing distance.
6. Personal motorised vehicule cannot be used to get to a lake, even partially.

A few less important precisions:

• Lakes may be crossed in any order.
• Crossing in one direction is fine. Although it is usually very impractical to do so, hence I will try to do both ways when possible.
• For big lakes crossings (>2-3km) a support boat is necessary. Therefore, private motor boats may be used, as security trumps every other rule.

Choosing the lakes

Which lakes should be considered ? Despite the Wikipedia page answering that exact question being created litteraly one week after I did most of my research, I think it is still important to specify what the rules are exactly. Indeed, there is not really a universal definition for what a lake is and wheter it is swimmable. Maybe you’re thinking I’m being pedantic, but it was in fact a headache to come up with the shortlist. Some “lakes” are old river beds, others are not swimmable, many of them are artificial, some are ephemeral, some are somewhat half way between a river and a lake… Anyway, the list might still change as the challenge goes on, but I will try to be as transparent as Oeschinensee and explain the methods and rules.

Using the 1:50'000 maps from swisstopo, we arrived to the following shortlist as an interactive map.

Or as a table.

Lakes that shall not be crossed

Another point of concern is that some lakes are more or less strictly forbidden to swim in. The Baldeggersee, Mauesee, Amsoldingensee and the Klignauerstausee are natural reserves and swimming is forbidden (or very restricted).

Lac de L’Hongrin is a reservoir and it is forbidden to swim in it. Actually, after talking with some people and gathering information here and there, most reservoirs are forbidden to swim in. But, this is typically for legal reasons. I will do my best effort to ask the electricity companies before, to let them know and learn how to minimise the risk. But they don’t always answer, so I reserve myself the right to breach some of those limitations. Please don’t sue me.

I took the liberty to exclude some “lakes” which, given their surface area, would have been in the 100 largest in switzerland. The Häftli and the Alter Rhein are both old river beds of the Aare and the Rhine, respectively. I don’t think many people consider them as lakes, and they would have been a bit boring from a crossing point of view.

Finally, sometimes, tragically, spontaneous lakes appear out of nowhere but shall not be considered, for obvious reasons.

Further pedantry

First, the areas aren’t exact, they won’t match what you would find on wikipedia. This is a consequence of a well known phenomenon called the coastline paradox. The larger your unit of distance, the rougher your shorelines will be, and therefore your measures will differ. For fractal-like things like shorelines, this means that there is no well-defined length. Basically, the fact that the map I’m currently using is quite rough, it will “cut corners” and therefore compute the wrong areas. But this should not impact too much the ordering of the lakes (perhaps the smaller ones are a bit shuffled around, but sometimes in life you have to cut corners to get things done).

The second point is that lakes move more than we think. From my experience, the advertised areas are upper bounds of the area of the lakes. First you have the reservoirs from hydroelectric plans, which size fluctuate in size to the rythm of people querying chatgpt to plan their next trip to the maldives. Furthermore, lakes (glacier lakes in particular) also fluctuate naturally and skrink drastically at the peak of summer time. I didn’t really think much further on this point and trusted the map. Hopefully I won’t have to cross a lake by jogging it though.

Also, note that lakes that are shared with neighbouring countries also count, otherwise we would get rid of three of the four largest ones. However, to keep it somewhat Swiss, the start or end of the swim must be in Switzerland.

When does it start? And how long will it take?

It has started on the 21st of April 2025. There is no hard deadline for when it should end but I will try to do it within the next two years. Feel free to remind me when I’m not making progress. I will do updates on this website and post my swims on strava.