The Sahara is the largest desert in the world. From north to south it is between 1,280km and 1,900km long, and about 4,800 km wide east to west. It covers most of North Africa, and more than a third of the African continent. It divides the continent into North and Sub-Saharan Africa. Its boundaries are the Atlantic Ocean on the west, the Atlas Mountains on the north, the Red Sea on the east, and the valley of the Niger River on the south. The Sahel region begins at the southern edge of the desert.
The desert consists of readily distinguishable northern and southern sectors. The north, less arid, supports most of the people who live in the region and contains most of the desert's oases. The southern zone is almost totally arid and is inhabited by nomads, mainly the Berber, Tuareg, and Maures, and Nubians. They survive by nomadic pastoralism, hunting, and trading along the margins of the Sahara.
The Sahara is a desert of extraordinary topographical variety. Its features include shallow basins, large oasis depressions, gravel-covered plains, plateaus, mountains, sand sheets, dunes, and sand seas (ergs). The highest part of the desert is at the summit of Mount Koussi in Chad which is 3,415 m high. The lowest point of the Sahara is 133 m below sea level in the Qattara Depression in Egypt. In the Algerian portion of the Sahara which extends south of the Saharan Atlas for 1,500 kilometers to the Niger and Mali frontiers, there are immense areas of sand dunes. These occupy about one-quarter of Algeria’s territory. The largest such region is the Grand Erg Oriental, where huge dunes two to five meters high are spaced about forty meters apart. Much of the remainder of the desert is covered by rocky platforms called humud and almost the entire southeastern quarter is taken up by the high, complex mass of the Ahaggar and Tassili n’ Ajjer highlands, some parts of which reach more than 2,000 meters. Surrounding the Ahaggar are sandstone plateaus, cut into deep gorges by ancient rivers, and to the west a desert of pebbles stretches to the Mali frontier.
Temperatures in excess of 55° C have been recorded in parts of Libya. Rainfall is scant and erratic, some areas endure several years without any water. In common with other desert regions, storms in the Sahara can be extremely localized. Strong, unpredictable winds known as khamsin and sirocco are typical of its weather system. These winds can blow for days on end, bringing with them dust and sand, which cover everything in their path and reduce visibility close to zero. Powerful sandstorms are dangerous for anyone caught in them.
The Sahara is crossed by the Nile and Niger Rivers, which together support most of the desert's human population. There are numerous rivers that originate outside of the Sahara but then enter the Sahara through underground waterways. In the southern part of the Sahara the tropical highlands of the Sahel get their water from the Nile River and the other rivers that flow to the Mediterranean. Other rivers feed into Lake Chad which is also located in the southern part of the Sahara. In the north, the greater part of the water flows from the Atlas Mountains and highlands of Libya, Tunisia, Algeria, and Morocco in the form of streams and wadis.
The desert is sparsely covered with various types of vegetation of types able to adapt to unreliable precipitation and excessive heat. These range from grasses, shrubs, and trees in the highlands to salt tolerant plants found in saline depressions.
Some of the mammal species found within the Sahara are the gerbil, jerboa, cape hare, the desert hedgehog, gazelle, baboon, hyena, jackal, sand fox, Libyan striped weasel, and mongoose.
Archeologists have found signs of human habitation going back 500,000 years, when the desert's climate was not as harsh as it is today. Fossils, rock art, stone artifacts, bone harpoons, shells, and many other items have been found in areas which today are considered too hot and dry to inhabit. Over 30,000 rock carvings of river animals such as crocodiles survive in total with half found in the Tassili n'Ajjer in southeast Algeria. Fossils of dinosaurs have also been found here.
The artifacts found were located near skeletal remains of giraffe, elephant, buffalo, antelopes, rhinoceros, and warthog, as well as the remains of fish, crocodiles, hippopotamuses and other aquatic animals which suggests that thousands of years ago water was quite abundant in the Sahara.
There is only one area recorded in the Sahara as a nature reserve, and that is Zellaf Nature Reserve in Libya with an area of 1000 km2.
References: http://www.worldwildlife.org/wildworld/ , http://library.thinkquest.org/