The lake of Alleghe in the valley of Cordévole is today exactly 242 years old. The moment of the birth of the lake is well known, at 7:02 in the morning of January 11, 1771 the river flowing through the valley became dammed by a landslide coming from the mountain Piz.
Fig.1. General view of the valley of Cordévole with the village and lake of Alleghe, on the left of the mountain Piz the scar of the landslide is barely visible in the forest, in the background the Civetta (3.220m).
The Alps-traveler Belsazar Hacquet (1739-1815) remembers a visit to the lake in 1780:
"The river Cordévole became my guide, by following him I would find the valley of Cadore. But only after some hundred steps the river was flowing in a large lake, existing here only for the last nine years. I walked around in eastern direction, to the villages of Sternade and Saviner until the mountain of Piz. First the lake was narrow, only by Saviner it became more than 100 venetian fathom [an old length unit used in the mining industry of these times, one fathom ca.1,8m] broad and more than thirty deep.
The last mentioned village once was situated on a hill, before it in a broad valley there were four smaller villages......which became flooded by the lake, but the fourth locality, named Marin, was buried with the village of Riete under the collapse of the mountain of Piz, last mentioned village situated previously on the top of the mountain."
Standing on the top of the mountain, I immediately noted that the mountain has a volcano on top of it, and it was possible to see how deep it went. After the mountain collapsed, it could be seen that its base was composed of limestone, build up by mighty layers, dipping from the west to the east with 45 degrees. The surface of the collapse is so plain, that a man has difficulties to climb on it to the mountain."
Fig.2. Historic depiction of the landslide-lake in the "Atlas Tyrolensis" of 1774 by Peter Anich and Blasius Hueber. Note the boulders on the southern shore of the lake, Anich and Hueber were one of the first cartographers to use signatures to display geomorphologic features in their maps (image in public domain).
The strange notion by Hacquet of a historic volcano in the Dolomites is based maybe on his discovery of volcanic rocks in the area, however - as we today know - these deposits are more than 235 million years old. At the time of Hacquet's geologic investigation volcanic forces were also believed to cause strong and sudden movements of the terrestrial surface, effects that maybe could also explain a sudden disaster, like a landslide. Notable how Hacquet describes the surface where the landslide "slipped away".
The landslide of Alleghe killed 48 people and destroyed parts of the village of Riete and some farm buildings. The landslide-lake inundated the village of Peron. Only in February 1771 a new river channel formed and so the lake as we today can admire it was finally grown up.
HÖFLER, H. & WITT, G. (2010): Katastrophen am Berg - Tragödien der Alpingeschichte. Bruckmann Verlag: 144