Water is a precious commodity. Folks in California are learning that the hard way in recent years. When you think about lower water levels, you probably think drought. But, there’s another natural phenomenon that can affect rivers and water supply. Large, violent volcanic eruptions.

We know massive volcanic eruptions toss aerosols way up into the upper levels of our atmosphere. These aerosols reflect more sunlight and can cause changes in rainfall patterns. Carly Iles and Gabriele Hegerl from the University of Edinburgh conducted a study to see just how much volcanic eruptions could affect rainfall, and more specifically, impact rivers.

The effect of large volcanic eruptions was more prounounced than previously thought. Illes and Hegerl looked over “observational records of streamflow volume for fifty large rivers from around the world that cover between two and six major volcanic eruptions in the twentieth and late nineteenth century,” according to the study’s abstract. The researchers grouped the rivers by region in order to better understand the how volcanic eruptions affected certain areas. Then, they used computers models to link rainfall with eruptions to predict which rivers were likely to be affected.

A regional pattern began to emerge. In certain areas, volcanic eruptions were followed by up to two years of reduced flow in some rivers. This was found in major rivers in mostly tropical regions including the Amazon, Congo and Nile. But, river flows increased in some subtropical regions including the south-west U.S. and parts of South America. Why the increase? Because of the disruptions of global atmospheric circulation patterns caused by these large volcanic eruptions.

“Our findings reveal the indirect effect that volcanoes can have on rivers, and could be very valuable in the event of a major volcanic eruption in future,” said Iles.

How this affects us

There’s a lot of factors at play here. Where does the volcano erupt? How big of an eruption is it? Which rivers are affected? A reduced flow in the Amazon isn’t as big of a deal as it would be on the Nile.

Plus, it’s not like these rivers just dried up. According to New Scientist, Illes estimates the Amazon river’s flow was reduced by around 10 percent. The drops are also temporary. Barring any other atmospheric changes, rainfall would return to normal and so would the river flows.

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