Clear water run-off and melting snow have filled this large natural basin.
I’ll be talking mainly about my two favourite temporary lakes in the Valserine valley: Lac du Creux au Niaizet in Lélex and Lac de la Tremblaine in the hamlet of Les Sept Fontaines. They can be deep in the middle and much shallower around the edges.
These temporary mountain lakes are found at altitudes of over 800m. Their crystal-clear water allows you to make out perfectly the diversity of nature at the bottom (dead leaves, stones, branches…).
What do these temporary lakes look like?
Their water is full of mineral particles that give them their green colour. Each particle corresponds to a particular colour. Slate particles give grey, iron particles give yellow and here the limestone particles give us this changing green from emerald to fir green.
But how do these lakes form?
These natural lakes require two main criteria: a counter-slope to prevent water run-off and impermeability of the underlying land.
These lakes were created by glaciers 10,000 years ago, when natural global warming caused the great glaciers to gradually melt. They have left a multitude of small basins in the valleys where water has accumulated. 96% of mountain lakes are glacial in origin, unlike tectonic lakes, where the collapse of land due to tectonic activity has formed basins that have filled with water.
In autumn, the lake is created by the rains, then goes into hibernation over the winter. The cold nights cause the surface water to cool considerably. A layer of ice forms, thickens and is covered by snowfall. Under this layer, darkness reigns and the water temperature remains at around 4°C.
But why can’t I swim there in summer?
At the beginning of March or April, the water warms up with the sun’s rays, the temperature at the surface of the lake rises and the ice cracks and then melts.
In early summer, the heat evaporates all this water.
And there… no swimming, but you can cross them, not by swimming… but on foot over the grass and pebbles, and you won’t believe that a few months earlier a lake covered this unspoilt nature!
Thematics
