Geoscience Reference
In-Depth Information
Carolina is on the order of ten to fifteen feet
per year. Clearly, thinning of these islands
cannot have been a long-term phenomenon,
or the islands would not exist today. Thin-
ning is the initial response of a barrier is-
land to sea level rise, and it will continue
until the island is narrow enough to begin
true migration, which can only occur when
the island is thin enough (90 to 180 meters,
or 100 to 200 yards) for overwash to fre-
quently cross the entire island in storms.
Typically islands in a full migration mode,
such as those at Cape Romain (South Caro-
lina), the Mississippi Delta, northern Yu-
catan, western Madagascar, and western
Turkey, are less than one hundred meters
wide. All these islands are responding in
significant part to sea level rise.
Today many deltas remain as population
centers. The two most populous deltas are
the Ganges-Brahmaputra (111 million in-
habitants) and the Mekong (47 million).
Deltas began to form when the rising
sea after the last ice age reached close to
the present sea level six thousand years
ago. Over the last two thousand years the
growth of deltas has accelerated, because
of the vast supply of sediment eroding
from farmers' plowed fields. More recently,
however, the land on most deltas has been
sinking. Localized maximum rates of sea
level rise on deltas range from four feet per
century in parts of the Mississippi Delta, to
2.4 meters (8 feet) per century on the Ben-
gal delta, to 3 meters (10 feet) per century
on some small deltas on the Pacific Coast
of Colombia, to as much as 10 meters (33
feet) per century on the river promontories
of the Nile Delta. The sinking of land caus-
ing such high rates of sea level rise is due
to the natural compaction of muds, often
exacerbated by oil and water extraction,
and the construction of dams upstream,
which reduce sediment supply. The con-
struction of canals on deltas removes
sediment-trapping marshes and mangrove
swamps that help to add land and elevation
on deltas. Sinking (and rising) may also be
related to tectonic forces within the earth,
particularly near active mountain ranges.
An example would be the river deltas on
the Pacific shores of Colombia and north-
ern Ecuador, which sink simultaneously
as the nearby Andes Mountain Range is
pushed upward.
Deltas:
Where rivers Meet the sea
Deltas are the bodies of sediment that
settle out when rivers flow into a standing
body of water like a lake or ocean. Viewed
from the air, the meandering and ever-
splitting river distributaries and the in-
tervening marshes and sandbars on deltas
form beautiful patterns such as those of
the Yukon Delta in Alaska and the Selenga
River Delta in Lake Baikal, Siberia. It is be-
cause of an abundance of water and fertile
land that many of the world's great civiliza-
tions originated on deltas, including those
on the Nile, Rhine, Indus, Pearl, Ganges-
Brahmaputra, and Tigris-Euphrates rivers.
