Okavango Delta: Botswana

The Okavango Delta in Botswana is a very large inland delta formed where the Okavango Riverreaches a tectonic trough in the central part of the endorheic basin of the Kalahari. All the water reaching the Delta is ultimately evaporated and transpired, and does not flow into any sea or ocean. Each year approximately 11 cubic kilometers of water spreads over the 6,000-15,000 km² area. Some flood-waters drain into Lake Ngami. The Moremi Game Reserve, aNational Park, is on the eastern side of the Delta. The scale and magnificence of the Okavango Delta helped it secure a position as one of the Seven Natural Wonders of Africa, which were officially declared on February 11, 2013 in Arusha,Tanzania.On 22 June 2014, the Okavango Delta became the 1000th site to be officially inscribed on the UNESCO World Heritage List.

The area was once part of Lake Makgadikgadi, an ancient lake that mostly dried up by the early Holocene. Although the Okavango Delta is widely believed to be the world's largest inland delta, it is not. In Africa alone there are two larger similar geological features: the Sudd on the Nile in South Sudan, and the Inner Niger Delta in Mali.

Floods

The Okavango is produced by seasonal flooding. The Okavango River drains the summer (January–February) rainfall from the Angolahighlands and the surge flows 1,200 kilometres in approximately one month. The waters then spread over the 250 km by 150 km area of the delta over the next four months (March–June). The high temperature of the delta causes rapid transpiration and evaporation, resulting in a cycle of rising and falling water level that was not fully understood until the early 20th century. The flood peaks between June and August, during Botswana’s dry winter months, when the delta swells to three times its permanent size, attracting animals from kilometres around and creating one of Africa’s greatest concentrations of wildlife.

The delta is very flat, with less than 2 metres variation in height across its 15,000 km².

Where the water goes

Every year, approximately 11 cubic kilometres (11,000,000,000,000 litres) of water flow into the delta. Approximately 60% is consumed through transpiration by plants, 36% by evaporation, 2% percolates into the aquifer system; and 2% flows into Lake Ngami. This turgid outflow means that the delta is unable to flush out the minerals carried by the river and is liable to become increasingly salty and uninhabitable, but this effect is reduced by the low salt content which collects around the roots of the plants. The low salinity of the water also means that the floods do not greatly enrich the floodplain with nutrients.

Salt islands

The agglomeration of salt around plant roots means that many of the thousands of islands have barren white patches in their centre, which have become too salty to support plants, aside from the odd salt resistant palm tree. Trees and grasses grow in sand near the edges of the islands that has yet to become too salty.

Approximately 70% of the islands began as termite mounds  where a tree then takes root on the mound of earth.