A number of mountain masses rise above the general surface throughout the plateau. Eastwards the surface slopes to the Kalahari Basin and northwards to the Etosha Pan. Much of Namibia's drainage is interior to the Kalahari. There are no perennial rivers within the country apart from those like the Okavango and the Cuando which cross the Caprivi Strip, and the Orange, Kunene and Zambezi which form parts of the southern and northern borders. Mean annual rainfall over the plateau increases northwards from less than 100mm on the southern border to over 510mm in the north-east.
Most rain falls during the summer, but is unreliable and there are years of drought.
Namibia possesses potentially the richest inshore and deepwater fishing zones in tropical Africa as a consequence of the rich feeding provided by the Benguela current. Measures are being taken to counter the effects of decades of over-fishing by both domestic and foreign fleets. Namibia also possesses scattered deposits of valuable minerals and its economy is dominated by the mining sector. Of particular importance are the rich deposits of alluvial diamonds which are exploited by surface mining, notably in the area between Oranjemund and Luderitz. Plant and animal life Both the Namib and Kalahari deserts are characterized by exotic, fragile desert plants.