The Okavango Delta (or Okavango Swamp), in Botswana, is the world's largest inland delta. It is formed where the Okavango River empties onto a swamp in an endorheic basin in the Kalahari Desert, where most of the water is lost to evaporation and transpiration instead of draining into the sea. Each year approximately 11 cubic kilometres of water irrigate the 15,000 km² area and some flood-waters drain into Lake Ngami. The Moremi Game Reserve, a National Park, spreads across the eastern side of the delta.

The area was once part of Lake Makgadikgadi, an ancient lake that mostly dried up by the early Holocene.

Geography

Flood

The Okavango Delta is a subject to seasonal flooding. The Okavango river drains the summer (January-February) rainfall from the Angola highlands 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 miles around and creating one of Africa’s greatest concentrations of wildlife.

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

Where the water goes

Every year circa 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 (flying ant) mounds, where a tree then takes root on the mound of earth.

Chief’s Island

Chief’s Island, the largest island in the delta, was formed by a fault line which uplifted an area over 70 km long and 15 km wide. Historically it was reserved as an exclusive hunting area for the chief. It now provides the core area for much of the resident wildlife when the waters rise.

Climate

Landlocked, far from the coast and surrounded by higher altitudes, Botswana is a dry country; the Okavango Delta is indeed an oasis in an arid land. It be a mistake though to think that the Delta's greenery reflects a tropical climate.

The Delta's average annual rainfall is 450mm, two thirds less than that of its Angolan catchment area, and most of it falls between December and March in the form of heavy afternoon thunderstorms.

December to February are hot wet months with daytime temperatures as high as 40C, warm nights, and humidity levels fluctuating between 50 and 80%. From March to May the temperature becomes far more comfortable with a maximum of 30C during the day and mild to cool nights. The rains quickly dry up leading into the dry, cold winter months of June to August. Daytime temperatures at this time of year are mild to warm but the mercury quickly plunges as the sun sinks. Nights can be surprisingly cold in the Delta with temperatures barely above freezing.

September to November sees the heat and atmospheric pressure build up once more as the dry season slides into the rainy season. October is the most challenging month for visitors - daytime temperatures often push past 40C and the dryness is only occasionally broken by a sudden cloudburst.

Wildlife

The Okavango delta is both a permanent and seasonal home to a wide variety of wildlife which is now a popular tourist attraction.

Species include African Bush Elephant, African Buffalo, Hippopotamus, Lechwe, Topi, Blue Wildebeest, Giraffe, Nile crocodile, Lion, Cheetah, Leopard, Brown Hyena, Spotted Hyena, Greater Kudu, Sable Antelope, Black Rhinoceros, White Rhinoceros, Plains Zebra, Warthog and Chacma Baboon. Notably the endangered African Wild Dog still survives within the Okavango Delta, exhibiting one of the richest pack densities in Africa. The delta also includes over 400 species of birds, including African Fish Eagle, Crested Crane, Lilac-breasted Roller, Hammerkop, Ostrich, and Sacred Ibis.

The majority of the estimated 200.000 large mammals in and around the delta are not year round residents. They leave with the summer rains to find renewed fields of grass to graze on and trees to browse, then make their way back as winter approaches. Large herds of buffalo and elephant total about 30,000 beasts.

Lechwe

The most populous large mammal is the lechwe antelope, with more than 60,000. A little larger than an impala with elongated hooves and a water repellent substance on their legs that enables rapid movement through knee deep water. They graze on aquatic plants and, like Waterbuck, take to water when threatened by predators. Only the males have horns.

Plants

The plants of the Delta play an important role in providing cohesion for the sand. The banks or levees of a river normally have a high mud content and this combines with the sand in the river’s load to continuously build up the river banks. In the Delta, because of the clean waters of the Okavango, there is almost no mud and the river’s load is comprised almost entirely of sand. The plants capture the sand, acting as the glue and making up for the lack of mud and in the process creating further islands on which more plants can take root.

This process is important in the formation of linear islands. They are long and thin and often curved like a gently meandering river. The reason for that is that they are actually the natural banks of old river channels which over time have become blocked up by plant growth and sand deposition, resulting in the river changing course and the old river levees becoming islands. Due to the flatness of the Delta, and the large tonnage of sand flowing into it from the Okavango River, the floor of the delta is slowly but constantly rising. Where channels are today, islands will be tomorrow and then new channels may wash away these existing islands.

credited to wikipedia