Considered as a whole, the Sahara is a rather low plateau with an uplifted center that has several mountainous areas. Elevations over much of the Sahara average only 1,000 to 1,500 feet (300 to 450 m) above sea level; in the central mountains a few peaks reach 10,000 to 11,000 feet (3,000 to 3,350 m). Except for the Nile and the Niger, which flow along the desert margins, there are no permanent rivers. There are numerous wadis, desert watercourses that contain water for only a short time after heavy rains.
The most familiar of the Sahara's features is the erg, an area of dunes and drifting sand. Although widely distributed, ergs occupy scarcely more than a tenth of the Sahara's total area. Less well known than ergs, but covering much more of the surface, are regs,stony plains from which all sand and other fine materials have been removed by the wind. Large areas of exposed bedrock that have been worn smooth by the wind are known as hammadas.