Oeschinensee was formed by several landslides on the flanks of the Fründenhorn and the Doldenhorn. The masses of debris that broke off on the Doldenhorn at 2100 meters formed a natural dam with a volume of well over one hundred million cubic meters.
The rock material is still permeable to water today, which is why Oeschinensee is drained underground. Oeschinensee has been part of the extended Swiss Alps Jungfrau-Aletsch UNESCO World Heritage Site since 2007.
An estimated 8000 years ago, large rockfalls occurred in Kandersteg, including from the Birre and the Fislisstock. The huge landslide from the Birre must have sealed off the valley basin of Kandersteg with a huge dam, behind which the Kander and the Oeschinenbach were dammed to form a lake until a mighty debris flow opened up the valley again.
In addition to the valley basin, Oeschinensee was formed around 3200 years ago due to several large landslides.
The masses of debris that broke off on the Doldenhorn at an altitude of 2100 meters formed a natural dam with a volume of well over one hundred million cubic meters:
The landslide masses of Dolden and Fisi also buried the basin between Kandersteg and Oeschinen, creating a kind of natural dam and allowing the Oeschinensee to build up with the melt and precipitation water behind it.
The sliding surface is very clearly visible today on the bare slabs above the Doldenhornhütte.
It is assumed that the landslides occurred at a time when the weather was significantly wetter than today and the glaciers were in a growth phase.
These and other factors could have led to an accumulation of these landslides.
The landslide mass is loose enough that the drainage of the lake is underground and the "Oeschibach" only comes to the surface a few hundred meters below.
This underground drainage is also the reason why the lake level is subject to large fluctuations throughout the year compared to other mountain lakes.
It is possible that the underground runoff has decreased somewhat over the years due to the input of glacial drift (very finely eroded rock material) by the surrounding streams, as the glacial drift seals very well.
This means that the lake could have increased in size very slowly but steadily due to the decreasing outflow.
It is very important to understand that our "eternal" mountains are also in motion at all times and therefore it must be expected that major rockfalls or landslides can occur in our time.
This is exactly what is currently happening at the "Spitze Stei" (a small remnant of the landslide at that time), where a much smaller landslide is emerging in relation to the 3200-year landslide.
You can find more information about "Spitze Stei" here.
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