Okavanga estuary

The Okavango Estuary is an endorheic inland delta in the Kalahari.

The Okavango is one of the few major rivers in the world that does not flow into the ocean, but ends in a vast inland delta within a closed (endorheic) basin in the Kalahari. Hydrologically and geomorphologically, this system forms an exceptional combination of an alluvial fan, seasonal floodplains, and a shallow aquifer, fed by a river with a long trajectory from the Angolan highlands.

The Okavango originates in the Angolan Plateau (Cubango/Cuito System), flows through the Caprivi Strip to Botswana, and reaches a tectonically controlled depression there at an altitude of approximately 930–1,000 m. At that point, the slope decreases sharply and the river is forced to branch into a dense network of distributaries, creating a classic alluvial fan.

In this basin, there is no permanent surface runoff to the sea; the system is endorheic. The water balance is dominated by evaporation and evapotranspiration: an estimated 96–98% of the annual inflow returns to the atmosphere via this route. Only about 2% leaves the delta as surface water via the Thamalakane and Boteti rivers, and a small portion infiltrates into regional groundwater.

Detailed studies show that infiltration plays an important local role: in some floodplains, 40–50% of the inflow is discharged into shallow alluvial aquifers via infiltration, after which the water is still largely lost through evapotranspiration. Direct evaporation from open water surfaces is estimated at approximately 9–12% of the annual inflow; the remaining loss is borne by vegetation and soil processes.

The reason the Okavango does not flow through to the coast is primarily structural-geological. The delta develops in a slowly subsiding graben, bounded by faults such as the Chobe–Gomare and Thamalekane–Kunyere faults. These structures form a local topographic depression in which sediment and water are trapped.

On a regional scale, this fits into the evolution of the Kalahari Basin, which formed since the Late Cretaceous through intracontinental downwarp and was later influenced by epeirogenic uplifts and the propagation of the East African Rift System into southern Africa. Movements along faults in the Pliocene–Early Quaternary blocked the originally southward flow of rivers such as the Okavango and Kwando, causing them to be diverted inland and eventually empty into endorheic depressions.

Hydrographically, the Okavango System is a typical endorheic basin: a drainage area without outflow to external watercourses or oceans, in which water leaves the system exclusively through evaporation, transpiration, and (limited) groundwater runoff. The “mouth” is therefore not a coastal delta, but an inland delta (alluvial fan) that fans out into an arid basin and gradually dries up.

On geological timescales, the endorheic character can vary: the system has experienced periods in the past with larger lakes (e.g., in the Makgadikgadi Depression) and possibly temporary connections with other drainage basins. On human timescales, however, the Okavango Delta behaves as a stable endorheic wetland, fed by an annual flood pulse that arrives with a delay due to the long travel time from Angola.

Pascal Brackman