The Paraná River behaves like a seismograph that records not the shaking of the ground but the pulse of the Sun. A century of daily water-level measurements at the gauging station in the city of Corrientes, 900 kilometers from the river's mouth, shows that when the Sun has many sunspots, the river floods more. This connection is neither accidental nor weak.

Astrophysicists from Buenos Aires compared two data sets: the Paraná's flow since 1904 and the international sunspot number. To separate slow changes from fast ones, they smoothed both series over 11 years, which is roughly the length of one solar cycle. On this timescale, the correlation reaches 0.78. For comparison, the correlation between a person's height and weight is usually around 0.8.

But the river also has a second, faster rhythm. If you subtract the 11-year smoothing, what remains are year-to-year fluctuations. These almost synchronously track the El Niño index, which is the ocean surface temperature off the coast of Peru. Here the correlation is 0.65. The flow peaks of 1982 and 1997 coincided with exceptional El Niños. In 1997, 180,000 square kilometers ended up underwater, an area larger than Greece. Some 125,000 people had to be evacuated, and 25 died.

So solar activity does not simply heat the atmosphere uniformly. It redistributes moisture around the planet. When the Sun is active, southeastern South America gets more rain, while East Africa and the Yucatán Peninsula, on the other hand, dry out. The Paraná River, like a giant instrument needle, shows this difference.

Today these data could form the basis for forecasting floods months or even years in advance. For agriculture, hydroelectric power, and shipping in the Paraná basin, such a forecast is not abstract science but a matter of life and money. What remains is to check whether this connection repeats on other rivers in the region, such as the Iguazú, and over independent time periods. If it does, we will have another tool for reading the Sun's pulse from river floods.