The Tera River basin sits within a geological horst, a block of land uplifted between the Cabrera Valley Fault to the north and the Las Portillas Fault to the south, part of the Montes de Leon mountain range. The terrain is granite, gneiss, and volcanic rock. Construction of the Vega de Tera dam began in 1954 and ended in 1956. The dam wall stood 33 meters high and spanned 380 meters across, holding a reservoir that formed part of a larger system of artificial lakes and canals operated by Hidroelectrica Moncabril, a hydroelectric company. The trial that followed the disaster would reveal that a failure in the dam's design was the root cause. The wall was not built to last. It was built to generate power.

The collapse came without warning in the darkness before dawn on January 9. At an initial peak discharge of 13,000 cubic meters per second, the flood traveled eight kilometers downstream toward Ribadelago. The village had no time, no sirens, no chance. Of its 664 residents, 144 died. Sanabria Lake, downstream, rose by 2.3 meters as floodwater and sediment poured in, though most of the sediment was deposited along the Tera River gorge itself. The remote location of the village meant the first rescue teams did not arrive until the following morning. Those who survived found their community destroyed, their neighbors and livestock buried under mud and debris. In a landscape defined by its beauty, the lake and the gorge that morning told a different story entirely.

In December 1959, seven employees of Hidroelectrica Moncabril and three from the construction company were indicted by the Court of Valladolid. A trial in March 1963 in Zamora concluded that the company owed 19,378,732 pesetas. The court sentenced the managing director, two engineers, and one expert to one year in prison for reckless negligence. The sentences were appealed, and the convicted were ultimately acquitted or pardoned by the state. Compensation to the families was grimly calculated by age: the equivalent of 600 euros for each deceased adult, 450 euros for those over 16, 300 euros for children under 10. The injured received 18 euros each. Hidroelectrica Moncabril was eventually absorbed by Union Fenosa. Many survivors abandoned the mountains entirely, migrating to the cities of Zamora and Benavente.

The Sanabria Lake Natural Park remains one of the most beautiful corners of the Province of Zamora, its glacial lake the largest in Spain of natural origin. The Tera River still flows through the gorge, past the place where the dam once stood. Ribadelago was partially rebuilt as Ribadelago Nuevo, a planned village on higher ground, though it never recovered its former population. The old village site, Ribadelago Viejo, still bears the scars. Geomorphological studies have mapped how the flood reshaped the gorge and the lakebed, depositing most of its sediment load before reaching the lake itself. The disaster remains a painful memory in Zamora, a reminder of what happens when infrastructure is built cheaply in terrain that does not forgive mistakes. Under Franco's Spain, the story was suppressed. The families had to fight for even the meager compensation they received. Today, local memorials mark the anniversary each January, refusing to let the 144 dead be forgotten.