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.
Around 3200 years ago, in addition to the valley basin, the Oeschinensee was also formed due to several large landslides.
In the process, the debris masses that broke off at the Doldenhorn at an altitude of 2100 metres, with a volume of well over a hundred million cubic metres, formed a natural dam:
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 gliding surface is very 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 lake is drained underground and the Oeschibach only comes to the surface a few hundred metres 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 removed rock material) from the surrounding streams, as the glacial drift seals very well.
This means that the lake could have grown larger very slowly but steadily because of the decreasing discharge.
It isvery 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 even in our time.
This is exactly what is currently happening at 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.