The lake arose in the last glacial (Würm) period, about 20,000 years ago, when a northern offset of the Drava glacier melted in situ and left dead-ice with extended fluvial deposits at its western rim damming meltwater.
About 20,000 years ago, during the last glacial period (Würm), a northern movement of the Drava glacier caused it to melt in situ, leaving behind dead ice with extensive river deposits that formed a dam on the west side, creating the lake.