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Thursday, 5 February 2015

The Lakes of Ounianga in the Sahara Desert, Chad

Lakes of Ounianga are a series of  18 lakes in the Sahara Desert, in North-Eastern Chad, occupying a basin in the mountains of West Tibesti and Ennedi East. The names of the lake groups are derived from the name of a village nearby. It was added as a UNESCO World Heritage site in 2012, the lakes are in a hot and hyper-arid desert that features a rainfall of less than 2 millimeters a year. The lakes are situated in a shallow basin below sandstone cliffs and hills, from where the ancient water flows.  Remarkably, this unique hydro-logical system is able to sustain the largest permanent freshwater lakes to be found in such an arid desert environment anywhere in the world. The lakes are remnants of a single large lake, probably tens of kilometers long, that once occupied this remote area approximately 14,800 to 5,500 years ago. As the climate dried out during the subsequent millennia, the lake shrank, and large, wind-driven sand dunes invaded the original depression, dividing it into several smaller basins.

Lakes of Ounianga are a series of  18 lakes in the Sahara Desert, in North-Eastern Chad, occupying a basin in the mountains of West Tibesti and Ennedi East.
Lake in the Ounianga Serir, northern Chad (© George SteinmetzCorbis). Image credit eMRaistlin Uth Majere

The 18 lakes altogether, in two groups about 40 km apart. They vary greatly in chemical composition, some being so salty that they can only support the most basic forms of life, while others are fresh enough to provide habitat for aquatic plants, fish and a diversity of other species.  The largest and most biologically important lake is Lake Teli and it has a surface area of 4.4km2 and a maximum depth of 10 meters.  Its water is fresh, and supports an abundance of life.  The sandy substrate is highly porous, so water flows freely underground between Lake Teli and 13 other smaller lakes in the eastern group.  Further west, across the dunes and sandstone ridges that characterise this part of the Sahara, the second group of four lakes (known as Ounianga Kebir) is found, dominated by Lake Yoan (3.6 km2 and 27 m deep).  This is a hyper saline lake which supports only algae and a few other micro-organisms.  Rocks around its shores are encrusted in white salt deposits, and a sprawling village of some 9,000 people is spread amongst the nearby hills and dunes.  There is a customs post, as this is the last habitable place on the main trans-Sahara truck route through to Libya. [source]

The lakes’ dark surfaces are almost completely segregated by linear, orange sand dunes that stream into the depression from the northeast. The almost-year-round northeast winds and cloudless skies make for very high evaporation rates; an evaporation rate of more than 6 meters per year has been measured in one of the nearby lakes. Despite this, only one of the ten lakes is saline. [source]

The reason for the apparent paradox—fresh water lakes in the heart of the desert—is that fresh water from a very large aquifer reaches the surface in the Ounianga Depression. The aquifer is large enough to keep supplying the small lakes with water despite the high evaporation rate. Mats of floating reeds also reduce the evaporation in places. The lakes form a hydrological system that is unique in the Sahara Desert. [source]

Ounianga Lakes in the Sahara Desert (NASA, International Space Station Science, 11/14/09)
Ounianga Lakes in the Sahara Desert (NASA, International Space Station Science, 11/14/09). Image credit NASA's Marshall Space Flight Center

Lakes of Ounianga are a series of  18 lakes in the Sahara Desert, in North-Eastern Chad, occupying a basin in the mountains of West Tibesti and Ennedi East.
Image credit Hannes Rada

Lakes of Ounianga are a series of  18 lakes in the Sahara Desert, in North-Eastern Chad, occupying a basin in the mountains of West Tibesti and Ennedi East.
Image credit Isabelle Boutriau

Lakes of Ounianga are a series of  18 lakes in the Sahara Desert, in North-Eastern Chad, occupying a basin in the mountains of West Tibesti and Ennedi East.
Image credit Hannes Rada

Lakes of Ounianga are a series of  18 lakes in the Sahara Desert, in North-Eastern Chad, occupying a basin in the mountains of West Tibesti and Ennedi East.
Image credit Hannes Rada

Lakes of Ounianga are a series of  18 lakes in the Sahara Desert, in North-Eastern Chad, occupying a basin in the mountains of West Tibesti and Ennedi East.
Image credit Isabelle Boutriau

Lakes of Ounianga are a series of  18 lakes in the Sahara Desert, in North-Eastern Chad, occupying a basin in the mountains of West Tibesti and Ennedi East.
Lagos de Unianga. Chad © George Steinmetz/Corbis/Cordon Press. Image credit AXA

Lakes of Ounianga are a series of  18 lakes in the Sahara Desert, in North-Eastern Chad, occupying a basin in the mountains of West Tibesti and Ennedi East.
Image credit escandio

Lakes of Ounianga are a series of  18 lakes in the Sahara Desert, in North-Eastern Chad, occupying a basin in the mountains of West Tibesti and Ennedi East.
Image credit IUCNweb

Lakes of Ounianga are a series of  18 lakes in the Sahara Desert, in North-Eastern Chad, occupying a basin in the mountains of West Tibesti and Ennedi East.
Image credit Hannes Rada

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