Scientific American - USA (2020-05)

May 2020, ScientificAmerican.com 17

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E C O L O G Y

Deltas Rising

River deltas are gaining

ground... for now

Earth’s river deltas have long been

home to vital ports and wetlands. Despite

sea-level rise, these economic and eco-

logical hotspots have expanded on the

whole in recent decades, scientists have

found—but this trend is unlikely to last.

Geoscientist Jaap Nienhuis of Utrecht

University in the Netherlands and his col-

leagues have pinpointed almost 11,

river deltas in data from satellites and

worldwide field studies. Using computer

simulations, they estimated how tides,

waves and human activity upstream

affected each delta’s size and shape from

1985 through 2015. In many watersheds

along South America’s eastern coast, as

well as A sia’s eastern, southern and

southeastern coasts, the researchers

found that deforestation increased sedi-

ment carried by rivers—and that deltas

have grown accordingly. (The Ganges

delta, as seen here, is one such example.)

In North America, however, dams have

trimmed the volume of sediment avail-

able to nourish many river deltas.

Although sea levels rose about 10 centi-

meters during the period studied, river del-

tas in aggregate gained about 54 square

kilometers of new land each year, the

researchers reported in January in Nature.

But they say that growth is most likely only

a short-term phenomenon. Molly Keogh, a

wetlands ecologist most recently at Tulane

University, who was not involved in the

study, agrees; she notes that sea levels are

projected to increase about 60 centimeters

by the end of the 21st century. Also, delta

sediments are notorious for compacting

under their own weight over time.

This “double whammy,” as Keogh

describes it, will lead to an overall shrink-

age of river deltas across the world in

the coming decades. Beyond the loss of

low-lying infrastructure and fisheries, con-

sequences could include the inundation

of major population centers and agricul-

tural regions. — Sid Perkins

Ganges River delta

worldwide field studies. Using computer

affected each delta’s size and shape from

along South America’s eastern coast, as

well as A sia’s eastern, southern and

low-lying infrastructure and fisheries, con

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