Tiny particles that are ejected into the atmosphere by volcanic eruptions can change the water cycle enough to alter the amount of water in nearby rivers for several years.

Carley Iles and Gabriele Hegerl at the University of Edinburgh, UK, looked at records of streamflow for 50 major rivers after volcanic eruptions dating back to 1883. Following an eruption, the amount of water flowing through the Amazon, Nile and other rivers in many tropical areas dropped for up to three years. In other areas, such as southwestern North America, streamflow increased.

Volcanic particles in the air reflect sunlight back into space, which cools the surface below, shifting patterns of evaporation and precipitation.

Water managers may need to plan for such interruptions, because of the importance of rivers for water supplies.

Nature Geosci. http://dx.doi.org/10.1038/ngeo2545 (2015)