The dam was supposed to regulate the rate-of-flow of the river that it was near and to store 50 million cubic metres of water for agricultural and domestic use and for the tourism sector of the area.

[2] Signs of an imminent collapse began in November 1959; a "trickle of clear water [was] observed high on the right [side]" and then cracks were noticed later in the month in the concrete apron at the dam toe.

The breach created a massive dam-break wave, or wall of water, 40 m (44 yd; 130 ft) high and moving at 70 km/h (38 kn; 43 mph), destroying two small villages, Malpasset and Bozon, the highway construction site, and in 20 minutes, still standing 3 m (3.3 yd; 9.8 ft) high, reaching Fréjus.

[2] Various small roads and railroad tracks were also destroyed, with water flooding the western half of Fréjus and finally reaching the sea.

The death toll of the dam breach was reported as 423, with 135 children under the age of 15,[5] 15 minors between 15 and 21 years old, 134 men, 112 women, and 27 individuals who were never identified.

The event also ushered in the practice of posthumous marriage in France for civilians, as many women who lost their fiancés were granted the right to marry them after death.

As a consequence, water collected under a wall and was unable to escape through the ground due to the impermeability of the gneiss rock underneath the dam.

[8] Finally, another theory quotes a source stating that explosions during building of the highway might have caused shifting of the rock base of the dam.

Rain continued, and the dam guardian wanted to open the discharge valves, but the authorities refused, claiming the highway construction site was in danger of flooding.