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Monitoring lake Schottensee

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The inactive rock glacier lobe in lake Schottensee at Flüelapass, Canton Grisons

Swiss mountain permafrost research started in the 1970s in the talus slope above and SW of lake Schottensee at Flüelapass. Interesting information on the dynamics of mountain permafrost can still be obtained from this site today.

 

In its present form, the talus slope must have developed after the disappearance of the ice which covered Flüelapass during the last Ice Age. In 2002 SLF drilled two boreholes in which ground temperatures are measured at various depths. The existence of permafrost at the base of the talus slope could thus be confirmed. The high ice content in the talus slope indicates that during its formation, avalanche snow was covered by rock fall, thus leading to the formation of ground ice. The ice content was so high that the rock-ice mixture started to creep and a rock glacier was formed.
The rock glacier transported rock material downslope, leaving a shallow channel in its wake (Fig. 1). At the base of the talus slope and at the bottom of the current lake, lobe-shaped structures were formed. The photograph on Figs. 1 and 2 shows one of these lobes.
Nevertheless, for thermal reasons, permafrost cannot exist in a mountain lake. This therefore implies that the lake is more recent than the lobe at its base. This explains the presence of the marked depression in the lobe, close to the lake shore. The rising lake water melted the ice in the lobe, its surface collapsed and a so-called thermokarst depression was formed.

 

Formation of lake Schottensee

At the NW end of the lake, towards Davos, there is an area covered by very coarse blocks (Fig. 1). This is a rock fall deposit which most likely dammed the lake. Further evidence can be found at the lower end of this deposit. A spring emerges here and is surrounded by dune-like waves in the ground, so-called mega-ripples (Fig. 3).
These are formed by flood waves, which can result from lake outbursts. There was probably a breach in the dam after the occurrence of the rock fall and a flood wave flowed down the Flüela valley. The ripples can be observed very well from the uppermost parking place on the N side of Flüelapass (Figs. 1 and 3).