Saudi Aramco World: May/June 2014 - page 17

May/June 2014
15
beauty by what’s underfoot. Stone hammers, scrapers and other
tools are strewn about here, some dating from half a million years
ago to “new ones
from the Neolithic era “only” 5000 to 10,000
years ago. I meet Baba, his shortwave radio pressed to his ear, lis-
tening to the news, beside what looks like a field of wild grasses,
but it’s actually the reed bed on top of part of the lake.
“There are salt lakes in deserts worldwide, but freshwater
lakes you just don’t have,” says Baba.
The Sahara is the largest of the world’s hot deserts. (The
polar regions are deserts, too, and larger.) The sun here is so
intense that the record measured evaporation rate at nearby
Lake Yoan was enough to lower the water level by six meters
(19') a year. Yet now, and presumably during the last 3000
years of hyper-aridity, rainfall has been measured only in milli-
meters a year. Properly, any lake here “should become like the
sea with the highest density of salt,” says Baba, or it should
have long ago disappeared completely.
“But in Ounianga we have this very big freshwater lake,” says
Baba, beaming so warmly at the paradox that his smile could
accelerate the evaporation process.
Baba points to the
Phragmites
and
Typha
, the reed genera
growing up to six meters, floating on their own debris in the
20-meter-deep (65½') lake. “Here, the sun doesn’t hit directly
the water,” he explains. “The evaporation rate is very low, so
the water is just about perfectly fresh.” Plentiful fish and crus-
taceans testify to his point.
What we see here, he explains, is the result of a hydrologi-
cal system,
unique in the
world, that
keeps all but
one of the
11 lakes of
Ounianga
Serir fresh.
To under-
stand more,
we go on a
tour to the
central Lake
Tili. And, yet
again, we
have to step
back in time.
D
uring the wet millennia of the Humid Pe-
riod, water accumulated in what is now the
Nubian Sandstone Aquifer System, which
underlies much of the eastern Sahara. This is
the world’s largest fossil-water aquifer, and it
spreads roughly two million square kilometers (772,000 sq mi)
beneath Chad, Sudan, Egypt and Libya to a maximum depth
of 4000 meters (12,800'). The Lakes of Ounianga are supplied
Away from the reeds, the fresh water of Lake Boukou is both pleasing to the eye and
habitable to aquatic life,
right.
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