Most of them are salt lakes. Why are these lakes salty? When water is exposed to the surface in an arid and dry environment, it becomes salty because of the high evaporation rate. Some lakes in the Sahara evaporate about 6 meters of water per year.
A unique geological form is the Ounianga lakes in northeastern Chad. These lakes were formed thanks to the trade winds that gave them a unique shape. They are punctuated by sand dunes and are located on the site of a former single large lake that was there from about 15,000 to 5,500 years ago.
All 18 lakes of Ounianga are fed by water accumulated in an underground aquifer during a humid period in the history of the Sahara thousands of years ago.
They are not large reservoirs; their total area is only 20 square kilometers. Once there was a large lake here, on the banks of which life was vibrant. People established agricultural fields and orchards here. There is archeological evidence of attempts to stop the desertification of the lake area some 7,500 to 5,000 years ago. This information has been obtained both from satellite imagery and rock paintings describing the events.
The desert sand, which was increasingly blown onto the farmland, was attempted to be stopped by the ancient inhabitants of the area with V-shaped barriers. These barriers were erected in rows and on a large scale, testifying to the great determination and extraordinary organization of the people even in such remote times. Unfortunately, the technology of the time could not stop the forces of nature, and the people who lived around the lake were forced to leave the area.
In 2012, the lakes were included in the UNESCO World Heritage List.
