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Before the Rockslide
First Left Image: The area that collapses into the Rhine Valley (brown) is marked in red.

Second Left Image: A view from the Tschingelhörner into the Rhine Valley shows the section (marked in red) east of the future site of the Segneshut (Segneshütte) that will collapse. Together with the Nagens slope, it forms the Segnesvalley (Segnestal), through which the Flem River flows into the Rhine Valley.

After the Rockslide
First Middle Image: The rockslide deposits in red.

Second Middle Image: The left slope of the Segnesvalley (Segnestal) is gone. Below the Segneshut (Segneshütte), the Flem River carves a new path. The Rhine Valley is buried under debris. In the Segneshut area, deposits (yellow) remain, trapping the erosion debris carried downhill by surrounding streams. Over time, these deposits fill a natural dam, creating the Untere Segnesebene (light green).

The Rockslide
First Right Image: The mass of debris sets the groundwater-saturated valley deposits in motion. Before the rockslide, the Rhine Valley below the Flimserstein was filled with glacial and river deposits, much of it saturated with groundwater. A lake may also have existed in the area.

As the rock masses crash into the groundwater-soaked valley deposits, they push them aside, behaving like a thick barley soup spilling over the edge of a plate. This “soup” spreads through the Rhine Valley and into the Domleschg. If the hypothesized lake existed, its water would mix with the slurry, diluting it and aiding its spread.

Second Right Image: The valley deposits travel remarkable distances, potentially carrying larger rock fragments. For example, the rocks forming the hill at the “Pardisla” site in the Domleschg may originate from this event.

The Formation of Lake Ilanz (Ilanzersee)
The rockslide blocks the Rhine river, creating Lake Ilanz within a few years. This lake rises to an elevation of approximately 900 meters above sea level.

Left Image: The lake (blue), dammed by the rockslide deposits (red). This view is from Ilanz looking down the valley towards Reichenau/Chur.

The Catastrophic Emptying of the Lake
When the lake level reaches approximately 900 meters above sea level, the dammed water begins to find its way through the debris and erodes it. The rockslide dam becomes permeable.

One day, the dam breaks, and the lake empties catastrophically. Massive volumes of water rush downstream, carrying vast amounts of material with them. Within weeks, or possibly just a few days, an 80-meter-deep gorge is carved.

When the lake level drops to approximately 820 meters above sea level, this phase of rapid, catastrophic draining comes to an end.

A Slower Transformation follows
The second phase of draining is more gradual. The lake slowly empties in smaller steps as the Rhine carves a path through the debris. This slow, persistent erosion creates the unique river landscape known today as the Rhine Gorge (Ruinaulta).

Small Lakes Form on the Rockslide Deposits
Several small lakes form on the debris. The most famous are Lag Grond (Laax), Lake Cauma (Flims), and Crestasee (Trin).

The Size and Depth of Lake Ilanz
The Lake Ilanz stretched approximately 30 kilometers in length and was about 220 meters deep at its deepest point. In terms of area, it was roughly the same size as Lake Walen. It extended into the Surselva, reaching as far as Rabius. The lake does not exist anymore today.

If it were to form today, towns such as Trun, Ilanz, and Valendas would be completely submerged under water.