Riñihue is the lowest in the chain known as the Seven Lakes, and it never stops filling. The Enco River pours into it without pause, and the San Pedro River normally carries that water away, threading past Los Lagos, Antilhue, and Pishuinco before reaching the sea at Corral Bay. With the San Pedro blocked, the math turned terrifying. If the lake climbed above the first natural barrier of fallen earth, twenty-six meters high, roughly 4.8 cubic kilometers of water would be loosed onto a riverbed built to handle four hundred cubic meters per second. The chronicler Mariño de Lobera had described something similar after a 1575 earthquake on this same ground. History was preparing to repeat itself, only now a hundred thousand people lived in the path.

Plans were drawn to evacuate Valdivia, and many residents simply left, watching the river and waiting. The Chilean Army sent the Batallón Escuela de Suboficiales south from San Bernardo, a unit made largely of young conscripts, boys who arrived to dig drainage ditches by hand at the edge of a lake that might kill them all. Grateful locals gave them a name that stuck: the Batallón de Hierro, the Iron Battalion. Alongside them came hundreds of workers from the power utility ENDESA, the development agency CORFO, and the public works ministry. Twenty-seven bulldozers were brought in, but the mud near the dams swallowed them; the dykes that mattered most had to be shaped with shovels and human muscle.

The engineer Raúl Sáez of ENDESA took charge of an operation that the country came to call simply the Riñihuazo. The strategy was patience under pressure: not to stop the water, which no one could, but to let it out slowly enough that the river could survive it. Crews carved channels to lower the unstable dam meter by meter, racing the rising lake. They did not confine the work to Riñihue. Drainages elsewhere in the Seven Lakes were dammed too, throttling the inflow from every direction. Most of those temporary dams were later removed, though the one on Calafquén Lake still stands today, a quiet souvenir of a frantic season. When American officers proposed simply blowing the barriers apart with missiles fired from a helicopter, the Chileans refused. A sudden breach was the one outcome they were fighting to avoid.

By 23 June the main dam had been brought down from twenty-four meters to fifteen, releasing some three cubic kilometers of water in a controlled, grinding descent rather than a single catastrophic surge. Even tamed, the flood had teeth. Los Lagos, Antilhue, Pishuinco, and the riverbank neighborhoods of Valdivia took on water, and damage spread along the whole course of the San Pedro. But the city itself stood. Roughly a month after the works began, Sáez and his exhausted crews finished. They had not defeated the earthquake; nobody could. They had refused to let it claim a second, slower victory, and in doing so turned a looming catastrophe into a story Chileans still tell about what determination and a few thousand shovels can hold back.