Greatest Places Physical Geography: Dr. Cecil Keen, July 1997

Okavango

Geographic Description: Described as "the river which never finds the sea", the Okavango in northwestern Botswana disappears into a 6,000-square-mile maze of lagoons, channels, and islands. The river system annually brings more than 2 million tons of sand and silt into the Delta, yet less than three percent of the water emerges at the other end to either flood Lake Ngami or cross another 300 miles of the Kalahari, then to enter Lake Xau and the Makgadikgadi Pans.

The Okavango Delta, in the midst of the Kalhan sands, is Africa's largest and most beautiful oasis. The River Okavango, which rises in the highlands of Angola, never reaches the sea; instead its mighty waters empty over the sands of the Kalahari. Here the great Kalahari desert thirst is locally quenched in a blue-green wilderness of fresh water, with emerald reed beds and towering trees.

It is a natural refuge and giant water hole for the larger animals of the Kalahari. The water gives rise to many forms of life unexpected in a "desert": There are fish, crocodiles basking on the sands, and hippopotamuses and swamp antelopes feeding on the vegetation.

The Okavango is the last surviving remnant of the great Lake Makgadikgadi whose waters and associated swamps once covered much of the Middle Kalahari. It also is closely associated with the Kwando, Linyanti, and Chobe swamps and river systems to the northeast. It is thought that long ago the Okavango, Chobe, Kwando, and upper Zambezi waterways flowed as one massive river across the Middle Kalahari, to join the Limpopo River and then to the Indian Ocean.

The earth movements that created the rift of the Kalahari-Zimbabwe Axis impeded this flow, causing a damming back of the giant river, which resulted in the formation of a series of high and complex swamps. As the Okavango River left the humid highlands and entered the arid flatness of the Kalahari, it slowed and dropped its sediment load. Channels became blocked and the water sought other courses, continuing to deposit its sediments wherever it traveled. Over time, some 2 million tons of sand and debris were deposited over the Kalahari sands, creating the characteristic fan shape of the Delta.

The Okavango's waters still cut the characteristic fan shape of the Delta. The Okavango's waters still cut paths through this built-up cone and deposit their sand load, causing the channels to continue changing direction. Superimposed on these changes were the climate fluctuations of the last million years. In arid periods these complex swamps and waterways would recede; in wetter times the myriad channels may have combined to form one vast river flowing into a huge lake--the former Lake Makgadikgadi.

2 parallel faults now control the direction in which the Okavango River enters the Kalahari Basin, in an area called the Panhandle. Other faults also direct its exit from the Delta, flowing south into the ocean of sand. As the Okavango flows over the Gomare Fault, a continuation of the Great Rift Valley of east Africa, that runs southwest to northeast, the slope of the land breaks it up into numerous channels, which fan outwards over the Delta. These are blocked by 2 southern faults, the Kunyere and the Thalamakane, which redirect the Delta's myriad channels. The Thalamakane Fault acts as a 150-mile-long natural dam: Here the channels abruptly change direction and join to form one river, the Boteti, which flows eastwards through a break in the fault towards the Makgadikgadi Pan. A small channel, the Nghabe River, continues southwest toward Lake Ngami, serving as both inlet and outlet depending on the strength and direction of the annual floods.

The present Okavango is still connected to the Chobe-Zambezi River system via the Selinda Spillway. However, recent arid conditions have meant that these water courses are now seldom joined. The geology of the Okavango is still inherently unstable, as the faults continue to move and earth tremors occur. Channels become filled with sand and debris, and massive plugs of papyrus interrupt their flow. The pattern of drainage in the Delta will continue to change.

Interior drainage systems occur typically in arid areas where water evaporates to leave an accumulation of salts as a saline pan, as in Makgadikgadi and Lake Ngami. The Okavango is unique in that it forms a freshwater Delta, simply because it has several outlets. Even though their outflow comprises only three percent of the Okavango's inflow, this is enough to carry away most of the salts and keep the Delta's waters fresh. In fact there are 2 groups of outlets: west to Lake Ngami, and south and east to the Makgadikgadi Pan via the Boteti River.

Although the total drainage pattern in the Delta is complex, there is an underlying simplicity in the slow and regular pulse of water that flows down each year from the Angolan highlands. South of the Panhandle the Delta fans out for many miles. During dry periods it is estimated to cover at least 6,000 square miles, but in wetter years, with a heavy annual flood, the Okavango's waters can spread over 8,500 square miles of the Kalahari's sands. Deep water occurs in only a few channels, while vast areas of reed beds are covered by only a few inches of water.

The Okavango offers an oasis of habitat for prolific plant and animal life in a personified state of "balance in nature." Two plants dominate the Delta's perennial swamps: papyrus, a giant sedge (type of grass) which grows naturally only in Africa, and the willowy phoenix palm. They provide a fascinating record of recent changes in the limits of the perennial swamps. Papyrus, being a herbaceous species, responds more quickly to changes in water level than the phoenix, which is a woody species. The full extent of the perennial swamps along the Thaoge River, before it began to dry up this century, can be seen by the distribution of the phoenix palm, which extends much further south than the papyrus. Conversely, papyrus extends much farther east than the palm, along the Moanatshira system. This indicates the expansion of perennial swamps during this century. On the Boro River, the papyrus and phoenix occur in the same places, indicating that the extent of swamp areas has remained relatively unchanged in the central Delta during this century.

Inhabiting the waters of the Okavango are an estimated 35 million fish of almost 80 species. The most abundant, three species of bream, are preserved from excessive predation by crocodiles feeding on the tiger fish that would prey on the bream. Hippos flatten paths through the papyrus on their nocturnal forays to graze, allowing easier access for the sitatunga and antelope to traverse across the swamps during their daytime migrations. Belts of forest fringe the swamps with tall trees giving shade to large herds of larger game. Beyond the forest fringe the landscape forms an open savanna park land, and in these drier areas the greatest concentrations of game are accompanied by the predator families: lion, leopard, cheetah, hyena, and wild dog. It is in these forest fringes and savanna grasslands that elephants and giraffes can be found browsing with antelope of almost every kind, from buffalo, wildebeest, and kudu, to sable, roan and impala. Okavango is a delicate and unique example of dynamic equilibrium at work in nature. A place worthy of being called a "greatest place."

Amazon

Greenland

Iguazu

Madagascar

Namib

Okavango

Tibet


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