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Niger Inland Delta - background 
In central Mali the Niger is joined by its tributary, the River Bani, at the city of Mopti. There, both rivers form the Niger Inland Delta. Known as the Inner Delta, it is a maze of channels, swamps and shallow lakes in huge shallow depressions which fill and empty every year as the annual flood passes through.
Around 20 000 years ago, the Sahara covered a comparable area to what it does today. But during the subsequent wet period, it contracted to small surviving areas of North Africa. To the south, major river systems developed with extensive areas of fine sediments like those of the Niger Inland Delta and Lake Chad. The many mobile dunes were consolidated by the growth of natural vegetation. From around 3000 BC, however, the climate in the area of the northern tropic changed, and the land reverted to its arid state. The Sahara expanded again. On its southern fringe, the current Sahel Zone, there have since been regular periods of drought.
The ecosystems of the savannas and the traditional forms of human economic activity have adapted to these climatically induced droughts, which have led to periodic desert-like phenomena in one region after another. The vegetation and the human and animal populations have recovered time and again from these dry periods, which have generally lasted for several years. Desertification The Sahel is the transition zone between the Sahara desert and the tropical regions of Africa. Sahel comes from the Arabic 'sahil', meaning 'edge' or 'coastline'. The Sahel is the northern part of the transitional climatic zone, with precipitations of 150 to 450 mm, and 8 to 10 months without any rainfall. The area extends from the Atlantic Ocean across Africa to the Red Sea north of Ethiopia. The width varies from 300 to 500 km. With regards to vegetation, the Sahel consists of a semi-desert in the north and the Sahel savanna in the south. In this area, plants are predominantly briars, shrubs and single trees, mostly Acacia albida and Acacia senegal. More vegetation grows after rainfall, but plants wither very soon in the desiccated soil. The biggest problem of the Sahel region is desertification, that is the expansion of deserts into natural barriers, savannas and steppes. There are several reasons for this. In the past years, there have been huge fluctuations in the amount of precipitations, including severe droughts in the years 1912-15, 1941-42, 1968-74, and 1983-85. As a result, the ground silts up, the vegetation cover disappears and, without natural barriers, the desert moves forward, assisted by the North-East Trade Wind, which also dries up the ground.
Even if there was more rain, it would be difficult to cultivate the sandy ground again.
The plants grow very slowly on the sandy ground, and the inhabitants needs more and more firewood and forage for their animals. That is why even the last sparse natural cover disappears. Without a natural plant barrier, the desert sand moves forward without meeting any resistance. Huge amounts of sand blow into the steppes and prevent the plants from growing, even when it rains. Then people move into the southern steppes to find usable vegetation. The population increases enormously, with a consequent need for wood, fields and animal fodder. The desertification continues to spread. The Sahel has lost a belt around 100 km wide to the Sahara since the 1970s. Last update: 16 April 2013
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