Scattered settlements flank the River Dra on the edge of the Sahara Desert.

Sand dunes can reach several hundred metres (around 900 feet) in height and can be several kilometres (around 4 miles) across - big enough to appear in the 1km satellite imagery used in this atlas. Areas of higher ground, left covered only by stones and gravel, are called hammada. Examples include the Hamada du Dra in the western Sahara and the Syrian Desert.

Desert ground blown clear of sand is called "hammada".

Windborne sand is a strong erosive force, although occasional rains cause flash floods which can carve out drainage channels: steep gullies on the slopes of hills and mountains and broader, shallow drainage channels in lower areas. These channels, called wadis in the Sahara and Arabia, are dry for most of the time and usually peter out into the desert sands.

A dry river bed in the Atlas Mountains of North Africa.

Where adjacent mountains induce rainfall, watercourses may drain to a lake where the water becomes saline due to the high evaporation rate. Lake Van in Turkey is one such salt lake, as is the Great Salt Lake in Utah, USA. The Bonneville Flats to the west is a seasonal lake which floods occasionally, but is usually a salt flat. The largest salt flat, or playa, is the Salar de Uyuni in the Andean Altiplano.