The lake that nearly choked: the story as a comic strip
In the 1950s, Lake Annecy was turning murky. Nine panels to understand, with Lou and Malo, how it was saved and why its water is so clear today.
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Malo says You can see the pebbles right at the bottom!
Lou says Has this lake always been this clean?
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Not at all! In the 1950s, the houses along the shore pour their dirty water straight into the lake. The water quality gets worse.
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That water feeds tiny algae, which multiply. The lake turns green and murky: this is called eutrophication.
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A professor and a doctor raise the alarm, and the mayor of Annecy listens. On 15 July 1957, eight towns around the lake join forces to save it: SILA is born.
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From 1961 to 1976, a great pipe is built all the way round the lake. It catches the dirty water before it reaches the lake.
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The pipe carries that water away from the lake to a treatment plant built downstream, at Cran-Gevrier. Since 1997, the Siloé plant has done the cleaning.
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Since 1966, scientists have kept watch over the lake. They measure how clear it is with a black-and-white disc, lowered until it disappears.
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It worked! Today, the measurements rate the lake water as “very good” for clarity and nutrients.
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The lake isn’t clean by magic: people keep protecting it, every day.
Lou says So it’s all thanks to a giant pipe?
Malo says And to people who decided together!
A story in nine panels to understand why Lake Annecy’s water is so clear today. It takes two minutes to read, from about age 7.
For grown-ups: what the records say
The alarm was raised by Professor Hubault and Dr Paul-Louis Servettaz, and taken up politically by Charles Bosson, mayor of Annecy. On 15 July 1957, eight towns on or near the lake set up an inter-municipal body: the forerunner of today’s SILA.
The wastewater collectors were built around the lake from 1961 to 1976. The water is carried to a treatment plant downstream of the lake, at Cran-Gevrier, replaced in 1997 by the Siloé plant.
Scientific monitoring of the lake began in 1966, prompted by “the first signs of eutrophication”, according to the SILA-INRAE report. In 2023, nutrient levels in the surface layer stayed below the “very good status” threshold, and clarity stayed above the “very good ecological status” threshold.
To go further: why the lake water is so clear, the long version of this story, and how the lake was born, Lou and Malo’s other comic.
Sources
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