An Ice Age Excavator
Sanabria Lake, the largest glacial lake on the Iberian Peninsula, occupies a basin carved by an immense glacier during the Würm glacial period. This period, part of the last major ice age, saw a massive ice cap centered on the nearby Sierra Segundera that covered over 440 square kilometers. From this high plateau, outlet glaciers flowed radially outwards. Two of these ice rivers, descending from the Tera and the Segundera-Cárdena valleys, merged to form a colossal glacier right where the lake now sits.
At its peak during the Last Glacial Maximum, around 21,000 years ago, the ice here reached a thickness of approximately 300 meters. The immense weight and slow, grinding movement of this frozen river reshaped the landscape. Acting like a giant excavator, the glacier plucked and abraded the granitic bedrock, transforming a V-shaped river valley into the deep, U-shaped trough visible today. The final basin was so deeply excavated—a process called glacial overdeepening—that the lake's water now reaches a maximum depth of 51 meters, with the true bedrock floor lying even deeper. The total volume of water held in this glacially-carved basin is over 96 million cubic meters.
A Dam of Rock and Debris
The existence of Sanabria Lake depends on the deep basin but also on the natural dam that contains its water. As the climate warmed and the glacier began its retreat around 19,000 years ago, it left behind a massive ridge of rock, gravel, and sediment at its farthest extent. This feature is a terminal moraine, and it effectively blocked the valley's eastern end near the town of Puebla de Sanabria.
Meltwater from the shrinking glacier filled the basin behind this moraine, forming the lake. The full deglaciation of the basin was complete by about 12,000 years before the present, stabilizing the lake by the early Holocene. The landscape of the surrounding Sanabria Lake Natural Park is evidence of this icy past. The mountains are dotted with smaller glacial lagoons, cirques (armchair-shaped hollows at the head of glacial valleys), and other U-shaped valleys, all sculpted by the same powerful geological forces. The bedrock is primarily composed of granitic rocks like granodiorites, whose low solubility contributes to the water's high clarity and low mineral content.