Enormous quantities of sediment are deposited in the flood-plains traversed by the Amazon and its tributaries in times of flooding. Scientists have hitherto considered the sedimentation rate to be generally constant with time.
Research conducted jointly by the IRD, the Universities of Washington1 and California2 and the Bolivian National Meteorology and Hydrology Service (SENAMHI) of La Paz, on two Bolivian rivers shows on the contrary that such events are irregular and less frequent than has been thought. These results, just published in Nature, emphasize that, in this Andean-Amazonian foreland, sedimentation is closely dependent on the flood amplitude, in turn linked to climatic variability, and particularly to La Niña, the cold phase of the ENSO (El Niño Southern Oscillation).
Continental-scale rivers can transport enormous sediment loads. In periods of flood, a proportion of these is deposited on flood-plains. In the Amazon Basin, crossed by the Earth’s largest river, great volumes of such sequestered sediment accumulations occur. This is especially so in the Llanos, the Bolivian lowland flood-plains which stretch from the foot of the Andes. An estimated 100 to 150 million tonnes of sediment are deposited each year respectively in the Rio Beni and the Rio Mamore flood-plains. These are the two Andean tributaries of the Rio Madeira, one of the Amazon’s main tributaries and source of more than half the sediment load transported by that river.
Marie Guillaume | alfa
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