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The huge Flims rockslide in the Swiss Alps has been the focus of scientific study for over 100 years, whereas a nearby rockslide at Tamins that occurred only slightly earlier, is understudied. Field investigations of the Tamins rockslide has provided new insights in its history, as well as that of the Flims rockslide. These investigations, complemented by interpretation of LiDAR imagery, has allowed us to document the sequence of postglacial events in the Vorderrhein river valley. We confirm the hypothesis that the Flims rockslide induced a large debris stream that transported huge volumes of liquefied alluvium, as well as large rafts of rock slide debris (Toma hills), many kilometers both down the Vordderrhein and up the Hinterrhein, its major tributary. The older Tamins rockslide partially impeded and was overrun by the debris stream, dividing it into two lobes.

The Flims rockslide blocked at about 8,200 year BP the Vorderrhein river and so gave rise to the Paleo-Lake Ilanz. Shortly after the filling of the lake a severe flood event happened. The lake emptied not totally, but remained at a lower level for a long time. After important and perhaps even complete filling of the remaining basin with sediments the incision in the dam restarted and progressed in several smaller steps. Based on the detailed field works, the following sequence of the events can be envisioned: The Tamins rockslide occurred first, damming Lake Bonaduz; the Flims slide come down soon after and probably entered in this lake; at the same time the alluvium was mobilized and resedimented as Bonaduz gravel; Lake Ilanz was dammed to a level between 820 and 936 m for very short time; the dam broke partly and a flood happened; the remaining lake survived for a long time and was filled up; the Rhein river eroded the floodplain down to the actual level.