Abstract
The Okavango Delta is southern Africa’s largest wetland ecosystem and probably the most pristine large wetland ecosystem in the world. Alex du Toit was the first to recognize the role of faulting in the origin of the Delta, proposing that the Delta lies within a graben structure related to the East African Rift Valley system. The history of its rivers is more ancient, extending back to the breakup of Gondwana.
Two mantle plumes initiated the breakup of Gondwana: the 140 Ma Karoo plume and the 130 Ma Parana-Etendeka plume. Domical uplift and rifting associated with these plumes created two major river systems: the Okavango-Zambezi-Limpopo system and the Vaal-Orange system. The climate at the time of breakup was hot and humid and the interior experienced extensive erosion, so much so that by the Oligocene (ca. 30 Ma), the sub-continent had been planed to base level, rising only a few hundred metres above sea level and mantled by thick, leached soils (now known as the African Erosion Surface).
Warping in the continent interior created uplifted arches and depressions, most notably the Kalahari basin. Arching severed the link between the lower Limpopo and its central African headwaters (Zambezi-Okavango), and a large lake formed in the Kalahari depression (Lake Palaeo-Makgadikgadi). This lake gradually disappeared, partly due to sedimentation but mainly due to the increasing dry climate.
The East Africa Rift system commenced in the Afar about 30 Ma ago and began to propagate southwards. In southern Malawi and especially in Zambia, the path of rifting has not yet been clearly established and the region is characterized by numerous horsts and grabens. One of these grabens passes through the Okavango Delta. The formation of these grabens has profoundly affected the courses of the rivers in the region.
The Okavango River debouches into the graben forming a large alluvial fan. Lakes have periodically existed at the toe of the Okavango fan where it abuts the bounding fault scarps, but these are not permanent. Some Okavango water discharges across the bounding fault scarps and flows into the Makgadikgadi depression to the southeast.
The Okavango River catchment is largely underlain by Kalahari sand, which forms the major particulate sediment carried by the river. Consequently, sediment carried by the river is mainly fine sand, with little silt and mud. The dissolved solid concentration in the river water is low (ca. 40 mg/L) and consists mainly of silica and calcium and magnesium bicarbonates. However, the volume of water entering the Delta each year is large and hence the solutes constitute the largest component of the sediment carried into the Delta.
The Okavango River discharges onto the alluvial fan where water is carried in channels that form the major primary distributaries. Channel margins are formed by vegetal material and are permeable, leaking water which sustains permanent swamps in the upper portion of the alluvial fan. The arrival of the seasonal flood increases the rate of channel leakage, forming the seasonal swamps on the lower fan. The advance of the flood water across the seasonal swamps is slow, as much of it infiltrates the ground, taking four to five months to traverse the 250 km length of the fan.
Bedload is confined to channels and as water leaks through the channel margins, channel beds aggrade, increasing leakage, which further promotes bed aggradation. Channels eventually fail and water diverts elsewhere. Channel formation and failure results in a constant shift in the distribution of water across the Delta surface. The demise of a channel system results in desiccation of the surrounding peat, which is then destroyed by slow-burning peat fires. Nutrients and fine particulate material accumulated in the peat is released, enriching the soil.
Most of the water delivered to the Delta annually is lost to the atmosphere by evapotranspiration because of the semi-arid climate. Transpiration of groundwater by terrestrial plants is the dominant means of water loss. The high transpiration rate of trees is particularly important: trees grow on islands and their transpiration lowers the water table beneath islands so there is a net flow of water towards islands. Solutes, especially silica and carbonates of calcium and magnesium, precipitate, leaving only the very soluble sodium carbonate in solution. Its concentration rises and impacts on the vegetation, resulting in a zonation in the distribution of plant species on islands. The salinity of the groundwater ultimately rises to the point where gravity induced advection occurs, thus transferring the sodium carbonate to the deep groundwater. This process prevents the formation of surface saline brine in the Delta and surface water remains fresh. The accumulation of precipitated solutes results in expansion and this form of chemical precipitation is the major mechanism of sedimentation in the distal regions of the Delta. Islands are mainly initiated as a consequence of termite activity during dry periods. Sand ridges which form by channel bed sedimentation may also result in islands.
Constant changes in the distribution of water across the fan due to channel failure have profound effects on the ecology of the Delta: regions of swamp may revert to dry land, when rain flushes accumulated salts from the island soils; and formerly dry areas become seasonally or even permanently flooded. Such constant changes, operating on time-scales of decades to centuries, underpin the immense habitat diversity of the Okavango Delta.
