The Illgraben has one of the most active debris flow torrents in the Swiss Alps. The Illgraben catchment (erosion slope) measures an impressive 10 km² and consists of quartzite, limestone deposits and dolomite. The latter in particular is especially susceptible to weathering and yields a lot of material that is swept along in the debris flow.

For this reason, new stone is constantly being found in the south-western part of the erosion slope. The light colour of this stone sets it clearly apart from the surrounding area. Since there have been regular debris flows in the last 100 years, the Rhone could never be canalized in the Pfyn-Finges Forest region, resulting in the unique floodplain landscape we see there today. On the scree slope itself, a scrubland has developed that is unique in Switzerland.

The Illgraben in action