Last year Ohio State University geophysicist Michael Bevis and his team were studying Global Positioning System (GPS) data from a reservoir lake in Chile when they spotted something "really strange": an oscillation of the lake bed. "We realized that whenever the lake was being filled," Bevis says, "the ground was going down." When the water was removed, the ground bounced back.
Scientists have known for a long time that Earth's surface rises and falls, much like an elastic spring, from the weight of the atmosphere. But Bevis found that rock also compresses and rebounds with the changing weight of water. The oscillations had been hidden in GPS data for many years, overlooked by researchers who had dismissed it as "noise."
The discovery, announced recently in Geophysical Research Letters, was an unexpected by-product of Bevis's interest in South American geology. To that end, he installed GPS stations throughout the continent. One, at Manaus, Brazil, near the confluence of the Negro and Amazon rivers, helped confirm his findings at the reservoir. "It's not one of our major stations," he says. "We'd never paid it a lot of attention, but when we looked, we found this huge oscillation." That led to another discovery: The Amazon has up to three inches of seasonal movement, three to nine times the oscillation observed anywhere else.
Bevis's findings should make it easier to determine how much water is available in a river at any given time. "You can assume what the sort of [elastic] structure is [in the crust] and use it to weigh the water cycle locally," he says. "It's just like Earth is a big bathroom scale."