Unseasonable rains deluged Peru’s northern coast in 2017. Months of downpours unleashed cascades of mud and water, washing away neighborhoods and killing 162. Tens of millions were left to marvel at — or bemoan — the water’s force.
But Jorge Abad had a different take. Poring over aerial photos of rivers overflowing their banks and burying crops under a layer of silt, he says, “It’s not the water — it’s the sediment.”
Most people think of rivers as waterways, but a river wouldn’t be a river without the particles of sand, soil and clay carried by the current. Fine sediments, suspended in the water, give rivers their muddy hue. Coarser particles sink to the bottom, where they scour the river’s course, cutting a deep canyon down a mountainside or forming lazy loops across a flat landscape.
For millions of years, huge amounts of sediment washing down from the Andes have fertilized flood plains on both sides of the range. On the west flank, occasional flows triggered by infrequent El Niño events allowed early civilizations to flourish. On the eastern side, annual pulses serve up nutrients for the Amazon basin.
