Chapter Seven

Meteorology

7.1   Lake Annecy is often pictured in tourist brochures as a beautiful still, blue lake and with all the appearance of timelessness and stillness. However, it is in fact a very recent creation in geological time, and has been constant motion since the day of its creation.  This is where meteorology takes over the story.

7.2    The lake was created around 65,000 years ago in the middle of the most significant meterological event of recent history, the last ice age, when glaciers hundreds of metres high carved their way through the alpine rock.  By the time these glaciers melted at the end of the ice age, between 18000 and 14,000 years ago, the lake had achieved a surface area of around 63 sq km, much larger than its 27 sq km today. 

7.3    Since then its water has been in constant circulation - a grand annual water cycle emptying by rivers Thiou, Fier and the Rhone, to the Mediterranean south of Arles, and returning in clouds blown across France from the Altantic whose rainfall feeds mountain streams of the Annecy basin, such as the Laudon, Bournette, Ire, Eau Morte, and Nant de Montmin, as well as a powerful underground source the Boubioz which enters the lake at a depth of 80 metres.

7.4   The basin’s catchment area is 251 km2, so the regions average annual precipitation of 1200mm (around double the average for France) replaces around 300 million cu m3 of the lake’s total volume of 1124 million cu m3  each year. In this way the lake water is refreshed on average once every 4 years.  The average length of time that water remains in a lake is a fundamental factor necessary to understand the physical, chemical and biological functioning of a lake, and not least its ability to cope with human waste and sewage deposited in it.

7.5    This grand circulation of water is the first of three distinct ways in which the weather keeps the lake’s water in constant motion and so contributes to refreshing and  maintaining the health of the lake.

7.6  Secondly, there is the action of the winds.  Due to the physical shape of the lake’s valley with high mountains either side, strong thermals are generated from the base of the valley which draw in strong winds and also create a propensity for sudden thunderstorms.   These same thermals on a clear day are a delight for parapenters who can be seen in their dozens circling around their launch site at Col de la Forclaz high above the lake.  The winds they generate are also a delight for the lake, since they do not just ruffle the surface of the lake as everyone can see with their eyes, but generate strong turbulence below the lake-s surface which circulates nutrients and oxygen to freshen up the lake.

7.7    A third and completely different source of movement is also caused by the weather, this time annually by the seasons, in a phenomenon referred to locally as brassage.  During the year most life takes place towards the surface of the lake where sunlight can penetrate far enough to enable phytoplankton to photosynthesize, and zooplankton to graze on the resulting harvest.  This in turn becomes food for increasingly bigger fish and birds.  Gravity then takes over as resulting excretia and dead zooplankton fall through the lake.  On their way down a portion of these nutrients are consumed by bacteria which use up oxygen dissolved in the lake to do so. So, over the year, nutrients fall through the lake to settle on the bottom and the lower lake water becomes deoxygenated.  This is where the weather steps in one more time in a fascinating and counter-intuitive way.

7.8   As we all know hot air rises and cold air falls.  So it must be the same for water.  (Every visitor knows from experience the warmest lake water is at the surface and the further down you go the colder it gets.)  So there is a column of water in the lake, cold at the bottom and warmer at the top. And as the lake gets warmer in the summer the temperature difference between the cold bottom and the warm top gets bigger, and as the lake cools towards winter the temperature difference gets smaller, but still with warmer water at the top and colder at the bottom.

7.9   So the cold water stays at the bottom with all its nutrients accumulating year on year, and the deep lake water continues to lose its dissolved oxygen year by year, until life at this level cannot exist.  Common sense.  But actually no, its not like this at all, because of a little known but crucial physical property of water.

7.10   Water achieves its maximum density at approximately 4 degrees centigrade.  Therefore the water at the very bottom of Lake Annecy, the heaviest water, is always at 4 degrees.  This is a layer of water called the hypolimnion (as opposed to the surface layer called the epilimnion and the layer in between called the metalimnion).  The water in the hypolimnion  cannot fall to 3 degrees or it would expand and rise up into the metalimnion above!  So the column of water in the lake always has a temperature at its base of 4 degrees.  As the lake cools towards winter the water column cools until it reaches 4 degrees top to bottom.  At this point the column becomes unstable and the action of the wind at the surface eventually tips over the column like a genga tower and there is a complete reversal (or brassage) with oxygenated water from the surface descending to  reoxygenate the lower depths of the lake, and nutrients from the same lower depths rising up to enrich the upper layers of the lake.  A lake management engineer couldn't have designed a more effective process for refreshing the lake annually.  It’s a small miracle at the bottom of the lake that was known to very few people, except of course divers such as Dr Servettaz who explored the bottom of the lake.

7.11   So these are the three ingenious ways in which the weather has over millennia maintained a constant circulation of fresh water, oxygen and nutrients into the lake to keep it healthy, clear and full of diverse life.  But not many local citizens paid much attention to the water cycle, or the effects of wind on the lake, or the annual brassage.  Not many people except for Dr Servettaz.  He could see that four years was rather slow for the water cycle and that even with all the weather's help, the lake was struggling to cope with something new, a rapid rise in human sewage discharged directly into the lake.

7.12   The three effects of the weather had been sufficient to cope with limited amounts of human effluent at the beginning of the century, but after the second world war the lake was beginning to choke.  It had become unable to cope with the sudden increase in waste  brought about by the rapid influx of population - people, as irony would have it, who were attracted to the area by the pure waters of the lake.

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