The Okavango swamps of northern Botswana are situated in the proximal reaches of a large alluvial fan (20 000km?), in the terminal depository of the Okavango delta. The river divides into a number of tributaries at the head of the fan, which sustain 6 000 km2 of permanent swamp. Lakes form an important morphological and ecological feature of this swamp. The lakes have arisen from re-flooding of old ox bows, or damming of water against the old meander ridge. Hydrology.