When the rivers in the Amazon basin flood their banks each rainy season, the added weight of the water causes land throughout the region to sink, scientists say.
At Manaus, Brazil, the land rises and falls as much as 75 millimeters each year, more than has been measured anywhere else on Earth, says geophysicist Michael Bevis of Ohio State University in Columbus. However, the variation doesn’t appear to be correlated with rainfall in Manaus itself, so soil swelling probably isn’t causing the elevation change.
The fluctuation is in perfect sync with the 15-meter annual rise and fall of the nearby Rio Negro, a major tributary of the Amazon, Bevis and his colleagues report in the Aug. 28 Geophysical Research Letters. When the Rio Negro rises and floods, Earth’s crust in the region sinks, they note.
Bevis and his colleagues accidentally discovered the synchrony in land and water motions while examining Global Positioning System (GPS) data collected from 1997 to 2002. Although GPS data have previously revealed changes in elevation near lakes and reservoirs as their water levels rise and fall, the data gathered in Manaus are the first to show such land-elevation oscillations caused by a body of flowing water.