896 21 AUGUST 2020 • VOL 369 ISSUE 6506 sciencemag.org SCIENCENEWS | FEATURES | MUDBangladesh
1094.China
1083.Colombia
143.Egypt
180Pakistan
123.United States
479.Brazil
755.Nile
180Total annual sediment
load of top 10 rivers
3860Amazon
755.
Tons annuallyBrahmaputra
739.Copper
218.Ganga
355.Indus
123.Magdalena
143.Mississippi
261.Ya n g t z e
376.Yellow
706.0200Change in annual sediment fux in tons (2000–10)Jhuo-shueiLenaOb
Biobío
AmazonYe l l o wYa n g t z eIndusMississippiXijiangThe human imprint
Around the world, mud cores drilled
from lake bottoms show a pattern
similar to that found in Lake Dojran
in Greece and Macedonia (right):
Sedimentation rates rose sharply about
4000 years ago as humans began to
clear landscapes. At Lake Dojran,
researchers used sediment levels of a
lithium isotope as a proxy for erosion.Roll on, muddy rivers
The Amazon currently tops the list of the world’s 10 largest transporters of
sediment to the sea. Other large muddy rivers help make Bangladesh and
China major suppliers of sediment.MUD ON THE MOVE
By David Malakoff; Graphics by Nirja Desai and Xing LiuHumans are reshaping the world’s mud supply, altering where—and how fast—it
piles up. Thousands of years ago, our ancestors caused erosion rates to surge by
starting to clear forests and plant crops, muddying lakes and rivers. Today,
deforestation and urbanization are causing some rivers to carry more sediment,
even as dams and efforts to curb erosion choke off sediment supplies to other
waterways. Such changes, together with precipitation shifts driven by climate change,
are leading to sometimes dramatic transformations in river deltas, coastal
mud flats, and the amount of mud that ultimately collects at the bottom of the sea.Clearer waters
Deforestation and farming increased sediment in some large rivers
from 2000 to 2010, particularly in South America. But dams have cut
loads elsewhere, especially in Asia. A 2019 study of 193 large rivers
estimated a 20.8% overall decline in sediment load. Here are the top
sediment gainers and losers.Plowed
feld Ocean foorFloodplain
sedimentSediment
fowCleared
Dam forest DeltasTidal
fatsTrapped
sediment
Deforestation
leads to larger deltas
Deforestation has increased
sediment loads in the
Amazon and other South
American rivers in recent
decades, helping expand the
continent’s river deltas by
some 16 kilometers per year.The (less) muddy
Mississippi
North America’s
biggest river has seen
sediment loads drop,
accelerating the loss of
its delta in Louisiana.Water moves vast quantities of sediment eroded
in highlands to the ocean, but human activities
can greatly boost or reduce sediment flows.600040002000 Present
years
agoErosion rate (relative scale)–01AmazonMississippiCopperParanaBiobíoMagdalenaCREDITS: (RIVERS MAP) HYDROSHEDS/NASA SHUTTLE RADAR TOPOGRAPHY MISSION; (TIDAL FLATS MAP) MURRAYET AL., N AT U R E, 565, 222 (2019); (EROSIONPROXY DATA) ROTHACKERET AL., SCIENTIFIC REPORTS, 8, 247 (2018); (SEDIMENTS DATA) LIET AL., SCIENCE BULLETIN, 65, 1, 62 (2020); JINREN NIPublished by AAAS