Lakes of Ounianga

Lakes of Ounianga are a series of lakes in the Sahara Desert, in North-Eastern Chad, occupying a basin in the mountains of West Tibesti and Ennedi East. It was added as a UNESCO World Heritage site in 2012.[2]

Lakes of Ounianga

UNESCO World Heritage Site
Ounianga Sérir
Location	Ennedi Region, Chad
Criteria	Natural: (vii)
Reference	1400
Inscription	2012 (36th session)
Area	62,808 ha (155,200 acres; 628.08 km^2)
Buffer zone	4,869 ha (12,030 acres; 48.69 km^2)
Coordinates	19°03′18″N 20°30′20″E
Location of Lakes of Ounianga in Chad

A 2009 astronaut's view of the Ounianga Sérir group. The image covers approximately 11 by 9 kilometres (7 by 6 mi). The dark surface of water is divided by linear, orange sand dunes that intrude into the depression of the lakes from the northeast.[1]

According to the UNESCO description, the lakes are in a hot and hyperarid desert that features a rainfall of less than 2 millimetres (0.1 in) a year. The lakes exhibit a variety of sizes, depths, chemical compositions and colorations.

There is a total of 18 lakes in groups as follows:

• Ounianga Kébir group: Lake Yoa, Lake Katam, Lake Oma (or Ouma), Lake Béver, Lake Midji, Lake Forodom;
• Lake Motro, about 30 kilometres (19 mi) southeast of Ounianga Kébir;
• Ounianga Sérir group about 45 to 60 kilometres (28 to 37 mi) southeast of Ounianga Kébir: Lake Melekui, Lake Dirke, Lake Ardjou, Lake Téli, Lake Obrom, Lake Élimé, Lake Hogo, Lake Djiara, Lake Ahoita, Lake Daléyala, and Lake Boukkou.

The total surface area of the lakes is about 20 square kilometres (8 sq mi). The largest of the lakes, Lake Yoa, has about a 3.5-square-kilometre (1 sq mi) area and attains 20 metres (66 ft) in depth.

The names of the lake groups are derived from the name of a village nearby (literally, Ounianga Kébir = "the great Ounianga" and Ounianga Serir = "little Ounianga").

Geography

These lakes form a hydrological system that is unique in Earth’s deserts. Ordinarily when water is exposed at the surface in highly arid environments it becomes saline due to a high rate of evaporation. In this case, even though the rate of evaporation from Lake Yoa is equivalent to 6 metres (20 ft) a year, the total lake depth is 25 metres (82 ft), unique physical factors combine to keep all of the lakes, except the central Lake Teli, fresh. First, water accumulated in an underground aquifer during the wet millennia is supplied to the lakes. Second, wind blown sand separates the basin into 10 lakes with Lake Teli occupying a lower position than the lakes around it. Third, thick mats of reeds cover the surface of the fresh water lakes where they slow evaporation, but are absent from the saline waters of Lake Teli. As a result, greater evaporation takes place from the surface of Lake Teli, keeping its water level low. This allows water from the adjacent lakes to flow through the permeable dune barriers into Lake Teli keeping their waters fresh.[3]

Panorama of Sérir

Connection to the history of humanity

The climatic history of the region is speculated to have a connection to human migrations at the end of the last ice age about 11,000 years ago. The desert returned to the area after monsoons diminished about 5,000 years ago. Investigation of a drilling core revealed 10,940 layers in 16 metres (52 ft) of sediments from the bottom of the Lake Yoa before reaching the ice-age desert floor, each layer corresponding to 1 year.[4] There is photographic evidence that shows early human settlers of the lakes attempted to stop the desertification that inevitably inundated their orchards and crops. During the humid period the early humans found the freshwater lake area to be fertile enough to grow crops and may have also domesticated cattle. Rock art found in many areas of the Sahara attest to that. However, a change in the climate slowly and eventually led to drought that continues to this day. As the desert wind from the northeast blew, it carried sand from the plateau above down through notches in the land, depositing it right where farmers were growing crops. Seeing that the sand would eventually smother their crops and orchards the lake dwellers attempted to stop it by erecting barriers. These barriers can be seen in aerial photographs at several places among both Ounianga Kébir and Sérir. They appear as series of “V’s” like this: VVVVVV with some as long as 2,300 feet across and in rows. The effort appears to be extraordinary for it would have required many people to erect the many layers of barriers that can be seen today. Unfortunately these “fences” failed and sand eventually claimed the area driving out the settlers anywhere between 7,500 and 5,000 years ago

References

1. NASA Earth Observatory, http://earthobservatory.nasa.gov/IOTD/view.php?id=41425.
2. "Lakes of Ounianga". UNESCO. Retrieved 3 September 2012.
3. Kröpelin, Stefan. "The Saharan lakes of Ounianga Serir (NE Chad): a unique hydrogeological system". Archived from the original on 2009-12-08. Retrieved 2009-08-28.
4. Johann Grolle, "Miracle in the Sahara: Oasis Sediments Archive Dramatic History", Spiegel-online, May 21, 2013, http://www.spiegel.de/international/world/new-sahara-research-the-lakes-of-ounianga-a-900518.html