The 6th edition of "Couleurs du lac", an event organised by the ASL and the commune of Nernier with the support of the Chablais Geopark, was held on 29 September at the Château de Ripaille. Its title, "Memories of the future of Lake Geneva", is an invitation to everyone to question what, in the past of Lake Geneva, could "inspire" today's actions designed to prepare this great lake for living tomorrow.
We all enjoy this familiar Lake Geneva every day: it seems to be there forever, eternal, blue, transparent and soothingly stable. It is benevolent, offering us sumptuous services free of charge. Then one day, in September 2021, the lake's beautiful colours suddenly disappear: the water, gorged with microscopic algae, is almost entirely brown. A nauseating smell spreads over the beaches. We no longer dare to swim. Many people whose tap water comes from the lake are wondering about its health quality. The whole press is talking about it, and then ... it's forgotten, just as the greenish, gloomy Lake Geneva of the 70s was forgotten. In the summer of 2024, the news hit again: several beaches on the upper lake were banned after an intense rainstorm disrupted the operation of the wastewater treatment plants. Shortly afterwards, toxic algae polluted a small coastal area. The question then arose: could the recent deterioration in the state of the lake, although temporary, be a sign of weakness in the ecosystem? In these moments of doubt, it is useful to question the past and therefore to question the scientists who analyse the memory of Lake Geneva with an eye to the future. The 6th edition of "Couleur du Lac" welcomed two speakers who are endeavouring to share this memory of the future with the public.
With "Lake Geneva: 800,000 years of history of a great lake", Walter Wildi, Honorary Professor of Environmental Geology at the University of Geneva, gives the audience a history of Lake Geneva based on regional geology. After all, Lake Geneva is just as much a 'geological object' as the mountains that surround it. It was built over a geological time span of 800,000 years, during major periods of glaciation and interglacial periods. The first traces of a lake can be found at Ecoteaux, near Palézieux. Walter is a captivating storyteller on an exciting exploration of space and time through at least 4 ice ages and as many interglacial periods. For 10,700 years, during the post-glacial period, a whole waltz of climatic fluctuations (particularly in temperature and rainfall) manifested themselves from the Neolithic period until 3,000 years ago, with periods of lower lake levels below the lake's outlet. Léman then became an "inland sea" that no longer fed the Rhône downstream of Geneva. Human settlements, the origin of the palafittes, were established on its shores. Beyond this extreme case, Professor Wildi shows us that the impact of the relationship between climatic variations and the levels of Lake Geneva on the flows of the Rhône is a well-established reality.
In "Carrot memory Jean-Philippe Jenny, a researcher at INRAE (Thonon-Chambéry), tells us how the study of sediment cores has enabled us to reconstruct the deoxygenation trajectory of the deep water layers of Lake Geneva. He explains the complexity of the mechanisms involved. Between 1980 and 2000, deoxygenation was the result of a combination of eutrophication* of the lake, a local phenomenon caused by the intensification of human activities around the lake, and the lack of complete mixing of the water mass, which in turn was the result of a global dynamic, climate change. Using calculations and modelling, our speaker confirmed that this lack of oxygenation will increase in the near future under the effect of climate pressure, despite the good control of eutrophication over the last 15 years. The water mass at the bottom of the lake, deprived of oxygen-generating circulation, will become increasingly isolated, as if imprisoned under a lid, and will become a kind of 'fermenter' whose biological life is limited to micro-organisms and which releases puffs of pollutants.
So what to do ? Ambitious action to reduce our local impact on the climate would certainly be useful, but it would have no direct effect on the state of the lake. No, because we can envisage strengthening the lake's resistance to the stress caused by climate change, in particular by taking action in the area that supplies the lake with water to reduce its pollution load even further. This area, known as the "catchment area", which stretches from the crests to the coast, is both vast (7,900km² from the Rhône glacier to Geneva) and diverse (mountains, forests, towns, villages, fields, crops, etc.). Yet it is possible to take action and mobilise stakeholders at this scale: the " Feedback on the "Clean Rivers" operation presented by Jean-Marcel Dorioz and Olivier Goy, proves it. In a nutshell, it all began 40 years ago when the CIPEL demonstrated that most of the lake's phosphate pollution, the main cause of the eutrophication referred to above, came from domestic wastewater effluents discharged insufficiently or untreated throughout the Lake Geneva hydrographic network. In response to this problem, the ASL took the daring initiative of launching an exhaustive participatory inventory of the uncontrolled discharges polluting Lake Geneva. This was the "Operation Clean Rivers", code name ORP, which was gradually rolled out over three-quarters of the Lake Geneva hydrographic network (i.e. over 8,000km of waterways). Over the course of a decade, almost 3,000 volunteers have been mobilised to cover this network. Each volunteer, individually or in a group, took charge of a stretch of river or stream, mapping it out, noting the state of the banks and the environment, noting waste, locating and, above all, identifying unconnected sewage discharges. The results of a simple chemical analysis kit and a multi-indicator observation sheet are used to determine the polluting nature of the discharges identified. This massive surge of rigorous observations in the field is used to help a region discover the extent of the damage to its aquatic environment, even where it was not expected. The results** were communicated to local and cantonal councillors so that they could remedy the polluting and illegal situations that had been identified and set up efficient wastewater collection and treatment systems (including phosphate removal). In this way, protecting Lake Geneva becomes everyone's business.
What's next? Safeguarding Lake Geneva, which is more than ever everyone's business, is a completely different matter. Admittedly, there are still some recalcitrant polluting pipes, accidental leaks, accumulations of waste... a whole series of recurring attacks on water quality that need to be brought more under control. But the major challenges facing Lake Geneva and the pressures associated with them lie elsewhere. The lake ecosystem is in the front line: 'reconfigured' by changing temperatures, it tends to react differently to pressures.
The nature, origin and spatial distribution of the pressures exerted on the lake have also changed: the pressures have multiplied, diversified, relocated and dispersed. Water pollution provides a good picture of the changes underway: in addition to the 'classic' pollutants such as heavy metals and PCBs, the pollution load now includes 'emerging' pollutants such as micro-plastics and the countless endocrine disrupters that are having a surreptitious impact on the ecosystem through group actions, in a 'cocktail' of micro-doses. Another critical transformation is that a large proportion of the pollutants come from just about everywhere in the catchment area and are mobilised in a diffuse way during rainfall. This makes controlling water pollution all the more complex, especially as the ultimate causes of water pollution, like most other pressures, are often rooted far outside the Lake Geneva area, or even on a global scale! In such a context, everything can seem lost in advance. However, we still have room for manoeuvre at local level, a possible lever for compensatory action: to further improve the quality of the water that feeds the lake and the aquatic environments associated with it. The relationship between the state of the lake and the state of the region is therefore, as it was at the time of ORP, crucial to the current and future health of Lake Geneva.
Elected representatives are paying close attention to these issues. The presence of Anne Cécile Violland, Member of Parliament, and Christophe Songeon, Vice-Chairman of Thonon-Agglo, at this sixth "Couleurs du lac" is testimony to this, not to mention the unfailing support of Marie Pierre Berthier, SIAC Vice-Chairwoman in charge of the Geopark, over the last 6 years. Of course, no progress would be possible without all of you who share, relay and support our ambition to preserve the long-term health of Lake Geneva.
Paul ROUX and Jean-Marcel DORIOZ (ASL committee)
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Our thanks go :
- the two guest speakers, W. Wildi and JP Jenny
- the Fondation Ripaille for its hospitality
- the town of Thonon for its encouragement
- to the town of EVIAN, the CGN and the Maison de la Rivière (Vaud) for their help
Nota bene :
* Eutrophication results from an over-supply of phosphates to the ecosystem, causing an overproduction of organic matter that pollutes the lake.
** ORP
The figures speak for themselves:
Number of rivers: 315
Total length of watercourse : 8,300 km or 16,600 km of riverbanks!
In 12 years, nearly 3,000 volunteers (families, school classes, scouts, etc.) and civil servants, students travel 12,000 km on foot and flush out :
20,143 pipes of which :
6,077 are polluting or suspect of being, whose
- 2,254 are unquestionably polluting
- 1,390 are very probably polluting
- 2,433 are probably polluting, to be confirmed
- 7,421 waste disposal sites
ASL publishes 985 files detailed results sent to 493 municipalities the Lake Geneva region, as well as the cantonal, departmental and federal authorities concerned
