Rivers vary greatly in size. The landscape in Namibia is crisscrossed by a number of rivers, and some of them are so small they dry up during hot, dry seasons for several years as a result of erratic rainfall. After heavy rains, these dry river courses are turned into raging torrents, racing down to the sea where they discharge their muddy load.
The underground water supply is recharged by these floods, providing a vital source of moisture for trees and animals during the dry periods. The majority of rivers in Namibia are ephemeral, flowing for a short time, sometimes only hours, after periods of high rainfall.
Rivers are also valuable to agriculture because their valleys and plains provide especially fertile land for growing crops. Farmers in dry regions use river water to irrigate their land.
The only perennial rivers in Namibia are shared with its neighbours; they are the Orange, Kunene, Okavango, Zambezi and Chobe. Each of these functions as a national frontier with limited irrigation potential. Furthermore, their waters are capable of enriching only the extreme northern and southern regions.
A number of other rivers flow intermittently and contain surface water only after the coming of rains and floods.
These include the Fish River, the westward flowing Swakop and Kuiseb, the shallower and eastward-flowing Omuramba, Omatako and Epukiro. There is good ground water though, beneath and around the riverbeds. What moisture remains above ground resides in shallow clay or limestone depressions, most of them in the sandveld semi-desert and some of quite prodigious dimensions. The largest of these is the world-famous Etosha Pan.
All of the rivers crossing the Namib Desert, including The Orange and Kunene, serve as 'linear oasis', allowing plants and animals not adapted to hyper-arid environments to thrive in the desert. Species vary from upstream to downstream in each river and from the southern rivers to the northern ones.
In the northern rivers, elephant and rhino use the rivers for food and water, but are not restricted to a single water course. Mammal species range from minute three-striped mice and tree rats to the larger gemsbok, steenbok, jackal, fox and hyena.
Klipspringer and baboon can be the centre of extensive attention in the Kuiseb, whilst giraffe and elephant have been the focus of attention in Hoanib. The towering clay castles of the Hoarusib River, dwarf desert-adapted elephant in their search for water.