896 21 AUGUST 2020 • VOL 369 ISSUE 6506 sciencemag.org SCIENCE

NE WS | FEATURES | MUD

Bangladesh1094.96

China1083.03

Colombia143.59

Egypt180

Pakistan123.56

United States479.53

Brazil755.28

Nile180

Total annual sediment load of top 10 rivers

3860

Amazon755.28Million

tons annually

Brahmaputra739.51

Copper218.2

Ganga355.45

Indus123.56

Magdalena143.59

Mississippi261.33

Yangtze376.33

Yellow706.7

0

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Jhuo-shuei

Lena

Ob

Biobío

Amazon

Yangtze

Indus

Xijiang

Yellow

Mississippi

The human imprintAround 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 riversThe 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 MOVEBy David Malakoff; Graphics by Nirja Desai and Xing Liu

Humans 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 watersDeforestation 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

feldOcean foor

Floodplain sediment

Sediment

fow

Cleared

forestDam Deltas

Tidal fats

Trapped sediment

Deforestation leads to larger deltasDeforestation 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 MississippiNorth 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.

Present4000 20006000yearsago

Ero

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ate

(re

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cale

)

–3

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Amazon

Mississippi

Copper

Parana

Biobío

Magdalena

CR

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ITS

: (R

IVE

RS

MA

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(2

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JIN

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N N

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Published by AAAS

Corrected 16 October 2020. See full text.

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21 AUGUST 2020 • VOL 369 ISSUE 6506 897SCIENCE sciencemag.org

Deltas of changeMuddy deltas that form where rivers meet the sea support rich farmlands

and ecosystems. A 2020 study of 11,000 deltas found that 9% lost

land from 1985 to 2015, whereas 14% added area. Globally, deltas grew by

54 square kilometers (km2) per year over that period, mostly in South

America and Asia. The leading land gainers and losers.

Shifting tidesTidal flats, vast banks of mud that flank many coasts, are key habitats

for marine organisms and seabirds, as well as important players in

the global processing and storage of carbon and nutrients. Asian nations

boast the largest total expanses (below), but other regions have

extensive flats (above). Reductions in sediment flows imperil some

flats; one recent global study estimated flats have shrunk by at least

20,000 km2 since 1984.

Indonesia14,416

China12,049

United States6622

Brazil5389

Australia8866

Canada6477

India5788

Parana

Lena

Rh

ine

-Me

use

Go

da

vari

Ga

ng

a-B

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tra

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ga

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ow

10

5

0

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Ne

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elt

a la

nd

ch

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(k

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ea

r)

Big barriersAsian rivers were once among the world’s muddiest, nourishing huge deltas and tidal flats. But those features are now threatened by a phalanx of huge dams that prevent sediment from reaching the sea.

A decadal declineAlthough the Nile carries one of the world’s largest sediment loads to the sea, dams across Africa now block up to two-thirds of the sediment that flowed downstream just decades ago.

River fow, in cubic meters per second (m3/s), based on average long-term discharge

Tidal fat area, in square kilometers (km2). Each dot represents a 1° grid cell for the period 2014–16.

~200,000 ~0

0 10 20 30

Lena

Ob

Yellow

Yangtze

IndusNile

Rhine Volga

Ganga

Brahmaputra

Mekong

XijiangSenegal

Niger

Congo

Zambezi

DA

TA

: (D

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TA

) N

IEH

UIS

ET

AL

., N

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7, 5

14 (

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56

5,

22

2,

(20

19)

Published by AAAS

Corrected 16 October 2020. See full text.

on January 7, 2021

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Mud on the moveDavid Malakoff, Nirja Desai and Xing Liu

DOI: 10.1126/science.369.6506.896 (6506), 896-897.369Science 

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