Each year as the Amazon river floods, it makes a large portion of South America sink several centimetres before rising again once the flood has cleared, according to research in this month's issue of Geophysical Research Letters.
They say that this is the first time anyone has recorded the movement of a landmass in response to river flow.
The researchers believe their finding could help calculate the amount of freshwater on Earth.
Knowing this would help researchers understand events such as droughts and floods, but would require similar studies in other parts of the world.
The study began when one of the researchers, Michael Bevis, now based at Ohio State University, United States, noticed that a global positioning system (GPS) station near a lake in the Andes was recording an 'up and down' motion. Bevis wondered whether the rise and fall of the lake’s water level were making nearby ground move too.
Other scientists had already measured similar movements in landmass elsewhere, the largest of which was just over one centimetre. They predicted that the greatest motion would happen underneath the largest river system in the world — the Amazon basin.
Bevis looked at a point near the centre of the Amazon basin, in Manaus, Brazil. There, he found that the bedrock rose and fell by about seven and a half centimetres — three times the amount predicted in 2004 by another group of researchers.
Douglas Alsdorf, Bevis's co-author, warns that the measurements are approximations, calculated using basic models of how water moves around the Amazon basin and data from a single GPS station. Adding that researchers are not even sure what the bedrock under the Amazon is made of.
But Bevis and Alsdorf think their findings might eventually help ’weigh’ the amount of water in the Amazon basin. If additional records were made around the globe — Alsdorf believes similar effects might be seen in Africa's Congo river system — it might be possible to estimate the amount of freshwater on the planet.