Running Water - The Changing Earth: Exploring Geology and Evolution - page 306

Geo-Focus

The River Nile and the History of Egypt

Egypt is well known for its remarkable an-
tiquities, such as the pyramids and sphinx
at Giza, the tombs in the Valley of the
Kings, and numerous temples and monu-
ments. However, Egypt also has some
fascinating geologic features, particularly
the Nile River. In fact, in no other country
has its development and prosperity been so
closely tied to a single geologic feature
(

a number of gullies called wadis do carry
water to the river following infrequent
rainstorms. The Nile is fl anked on the west
and east by desert, and the only agricul-
tural land is on the Nile's fl oodplain
(

Mediterranean Sea was closed off from
the Atlantic at the Strait of Gibraltar.
As a result, the sea dried up, leaving a
vast plain more than 3000 m below sea
level. The Nile and other rivers around
the Mediterranean basin responded by
cutting deep canyons as they adjusted
to this new lower base level. By the
Pliocene Epoch (about 5 million years
ago), however, the barrier at the Strait of
Gibraltar was breached and the Medi-
terranean began to refi ll, thus raising
base level.*
During the time of deep erosion,
the Nile cut a canyon about 10-20 km
wide and 2500 m deep near Cairo. Even
465 km upstream at Aswan, the canyon
was cut 200 m below present-day sea
level. As sea level rose, the ancient river
canyon became an arm of the sea, but
by about 3.3 million years ago, the river
began depositing gravel, sand, and
mud in its canyon and it took on an
appearance much like it has today.
Since the completion of the Aswan
High Dam in 1970, fl ooding on the Nile
has been drastically reduced, so now it does
not deposit mud on its fl oodplain each
year. As a result, Egyptian farmers have had
to apply chemical fertilizers to maintain
agricultural productivity.

Figure 2) and delta, at the Fayum
Depression, and at a few scattered oases.
Although people have occupied what is
now Egypt for hundreds of thousands of
years, Egyptian civilization took root there
about 5000 years ago and persisted for
more than 3000 years. Even when ancient
Greece fl ourished, Egyptian civilization
was already ancient. But it was a civilization
totally dependent on the annual fl ooding
of the Nile, during which fertile deposits
accumulated on the fl oodplain. Indeed, in
ancient Egypt, taxes were even determined
by the extent of the annual fl ooding. And,
of course, the Nile was also the main avenue
of commerce throughout the region.
The Nile is unique in that it has so
few tributaries and fl ows such a great
distance through a desert, but in most
other respects, it is like any other river. It
responds to short- and long-
term climatic changes, inter-
ruptions in the system where
dams are built, and changes
in sea level. During the Late
Miocene Epoch, about 6 or
7 million years ago, base level
for the Nile was much lower
than it is now because the

◗

◗

Figure 1). Although the Nile, at 6825 km
long, is the world's longest river, its dis-
charge of 1584 m 3 /sec makes it a modest-
sized river when compared with the great
rivers of the world. Nevertheless, it is the
lifeblood of an entire country.
The Nile River rises as the Blue Nile
in Ethiopia and the While Nile in Sudan,
which join and fl ow north into Egypt and
eventually into the Mediterranean Sea. If
it were not for the abundant precipitation
in the Ethiopian highlands, Egypt, which
is nearly all desert, would have no river
at all. Indeed, the Atbara River in Sudan
is its only other tributary. Once the Nile
enters Egypt it has no tributaries, although

Mediterranean Sea

Nile
delta

Sinai
Peninsula

Giza

* Actually, the isolation of the Mediterranean
from the Atlantic and its refi lling probably
occurred several times.

Fayum
Depression

Cairo

Red
Sea

Gulf of
Suez

Western
Desert

Eastern
Desert

Nile River
and its
floodplain

Luxor

◗ ◗

Figure 1 Satellite image of the Nile River as it fl ows north
through Egypt. The only arable land is on the river's fl oodplain and
its delta and at a few scattered oases. The fl oodplain measures
about 20 km across at its widest point.

Figure 2 At this location, between Luxor and Aswan, the
fl oodplain of the Nile River is only a few hundred meters wide.
The view is toward the Eastern Desert, which begins a short
distance from the river.