Weighing the Amazon River

Weighing the Amazon River has been accomplished by watching the rise and fall of the Earth's crust with a Global Positioning Service (GPS) unit over several years as the river floods and drains during its seasonal cycles. GPS, through its network of satellites and carefully staged series of signals timed with exquisite precision by atomic clocks, can provide information about the position at the Earth's surface with horizontal uncertainty of about 1 mm and a vertical uncertainty of about 9 mm. Repeated measurements made over several years yield velocity measurements for any spot to an accuracy of about 1 mm/year. Around the wide world, a typical land movement up or down will be about 2 to 10 mm/year. But in large tropical drainage areas, with huge volumes of water pressing down on a river channel and floodplain, the oscillation can be bigger. Indeed, the peak-to-peak amplitude reported in this present measurement amounts to 50-75 mm/year. When the river is heavy, the land sinks down. Later, when the river lessens, the land rebounds.Scientists from the Instituto Brasileiro de Geografia e Estatistica and the Instituto Nacional de Pesquisas da Amazonas (Brazil), and from Ohio State University, the University of Memphis, and University of Hawaii (U.S.), saw the biggest displacement in Manaus, Brazil. One of the researchers, Michael Bevis of Ohio State, said that they were surprised by the size of the oscillation.

Bevis et al., Geophysical Research Letters, 15 September 2005
Contact Mike Bevis at mbevis@osu.edu or Doug Alsdorf at alsdorf@geology.ohio-state.edu
See also www.mps.ohio-state.edu