Science - USA (2020-08-21)

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

Deltas of change

Muddy 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 (km^2 ) per year over that period, mostly in South

America and Asia. The leading land gainers and losers.

Shifting tides

Tidal 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 km^2 since 1984.

Indonesia

14,

China

12,

United States

6622

Brazil

5389

Australia

8866

Canada

6477

India

5788

Parana

Lena

Rhine-Meuse

Godava

ri

Ganga

-Brahmaputra

Vo l ga

Mekon

g

SenegalNigerMiss

issippi

NileCop

per River

Ye l l o w

10

5

0

Net delta land change (km

2 /year)

Big barriers

Asian 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 decline

Although 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 (m^3 /s), based on

average long-term discharge

Tidal fat area, in square

kilometers (km^2 ). Each dot

represents a 1° grid cell

for the period 2014–16.

~200,000 ~

0 10 20 30

Lena

Ob

Yellow

Ya n g t z e

Nile Indus

Rhine Vo l ga

Ganga

Brahmaputra

Mekong

Senegal Xijiang

Niger

Congo

Zambezi

DATA: (DELTA) NIEHUIS

ET A

L., NATURE

, 577, 514 (2020); (TIDAL FLATS) MURRAY

ET A

L., NATURE

, 565, 222, (2019)

Published by AAAS