Elusive, but ecologically vital, the 500 km (310 mi) Ugab River only flows above ground for a few days each year. Shown in this false-color satellite image, the subterranean waters underlying this ephemeral river are shallow enough in places to fill hollows and sustain wildlife populations that include the rare desert elephant and the largest free population of black rhino. Pink-granite inselbergs, islands of rock left behind after volcanic activity, form bizarre sculptures in the sandy riverbed. Some, known as "petrified ghosts," are eerie hollow structures, carved by erosion. Image courtesy of USGS.

Namib-Naukluft National Park is an ecological preserve - the largest game reserve in Africa - in the vast Namib Desert, a part of which may be seen in this high-resolution satellite image. Due to the orange color of the sand, which develops over time as the iron in the sand is oxidized, the Namib Desert is considered the oldest desert in the world. Coastal winds create the tallest sand dunes in the world here, with some dunes reaching 300 m (980 ft) in height. Image courtesy of USGS.

On the edge of the Kalahari Desert, sand dunes are encroaching onto once-fertile lands in the north. Healthy vegetation appears red in this false color satellite image; in the center, notice the lone red dot. It is the result of a center-pivot irrigation system, evidence that at least one optimistic farmer continues to work the fields despite the approaching sand. Image courtesy of USGS.

The Zambezi River is one of the great rivers of Africa. Originating in swampy wetlands in Angola in southwestern Africa, the river flows 2,736 km (1,700 mi) across the continent to the Indian Ocean. This highly detailed true-color satellite image shows the stark eastern edge of the floodplain. To the left of the edge, deep blue channels wind among green, shallowly flooded plains. To the right of the edge, the land is dry. The city of Kasane is situated along the edge of the flood plain near the bottom center of this image. The eastern edge of the flood plain is defined by the Mambova Fault, which elevated the land on its eastern side, providing a natural boundary for the flood plain. The Zambezi and Chobe Rivers cut channels across the fault. The triangle between the two rivers and the fault creates Impalila Island. A channel of water, called the Kasai, connects the two rivers in the flood plain. Floods on the Zambezi occur when heavy rains fall on the wetlands in Angola and Zambia. The water flows downstream and gets backed up at the Mambova fault. The river expands over the flat floodplain behind the fault until the waters meet the channel cut by the Chobe River in the south. During the annual flood, the build up of water from the Zambezi River overcomes the Chobe, and water begins to flow south into Lake Liambezi. Image courtesy of NASA.

The Orange River serves as part of the border between Namibia and the Republic of South Africa. Along the banks of this river, roughly 100 km (60 mi) inland from where the river empties into the Atlantic Ocean, irrigation projects take advantage of water from the river and soils from the floodplains to grow produce, turning parts of a normally earth-toned landscape emerald green. A network of bright rectangles of varying shades of green contrasts with surroundings of gray, beige, tan, and rust in this true-color satellite image. Immediately south of a large collection of irrigated plots, faint beige circles reveal center-pivot irrigation fields, apparently allowed to go fallow. This Namibian irrigation project occurs along a section of the Orange River where the waterway turns north on its general westward path to the sea, not far from the eastern margin of the Namib Desert. Due to local climatic conditions, grapes from Namibia, the primary agricultural product of this area, are often ready for market two to three weeks before those of the main grape-producing regions of South Africa's Cape. Image courtesy of NASA.