A Fossil Desert
In many ways, the Kalahari is more of a fossil desert than an actual desert. The great sand-covered plateau that includes the Kalahari is testament to desert conditions that once more closely resembled the Sahara than the current semiarid grassland mitigated by scattered acacia trees and shrubs, which fill the same niche that plants like mesquite do in the deserts of North America. The region is marked by massive sand dunes, generally running north-south and ranging in height from 20 to 200 feet (6–61 m). The dunes run parallel for 50 miles (80.5 km) or more, creating ridges separated by intervening channels. Such sand dunes in the Sahara to the north are actively growing and shifting, so that no plants can get rooted before they are buried by the shift of the dune. The Kalahari must have been built up under similar conditions, but at some point rainfall patterns shifted sufficiently that the winds pushing the dune moderated and the plants got enough water to put down roots and stabilize the dune surface. Therefore, the dunes of the Kalahari are now long, stable, grass- and shrub-covered ridgelines.
The dunes stabilized some 10,000 to 20,000 years ago, for reasons climate scientists do not fully understand. Curiously, while the end of the last ice age some 10,000 years ago created many of the modern deserts, in the Kalahari it apparently moderated desert conditions. That massive climate shift made conditions much more harsh in the Sahara and in North America turned grasslands into deserts. But here, the same shift apparently converted a raw sand desert into a semiarid grassland. The explanation probably lies in planetwide shifts in rainfall patterns relating to the accompanying warming, sea level rise, and shifts in trade winds, ocean temperatures, and monsoons. Although the Kalahari remained in a desert-prone latitude hedged in by rain-blocking mountains, enough of an enhanced wet season delivered plenty of rain to greatly soften the desert conditions.
As in most deserts, water remains the crucial limiting factor. The Kalahari receives an average of about 8 inches (203.2 mm) of rain per year, while the surrounding semiarid sand plains receive far more. Even so, the rain remains variable and seasonal, with some years yielding two or three times the annual average.
Most of the year, not a single stream, river, lake, or pond breaks the desert surface. The only permanent surface water is the Boteti River, flowing out of the great swamps of the Okavango delta at the northern edge of the desert in northern Botswana. Here, hippos and a variety of aquatic birds and animals provide a rich resource. Heavy rains throughout the desert can cause periodic floods, but dry stream channels quickly deliver the floodwaters to flat low-lying depressions with no outlet. The water collects into shallow, fleeting lakes before evaporating and leaving behind dissolved salts and minerals. As a result, these low-lying salt flats or pans resemble the same features found in almost all deserts.
Several lakes on the edge of the desert, including Lake Xau and Lake Ngami also sometimes collect the floodwaters that heavy storms send crashing through the Kalahari, as well as the flow of the Boteti River, much of which is carried underground. The lack of permanent water means the Kalahari has only a handful of native fish species, but that includes the remarkable Brooder Fish, which compensates for the hard and uncertain times by keeping its eggs in its mouth until they hatch, whereupon the mother tenderly spits her hatchlings out into an uncertain world.