ENVIRONMENTAL-CHANGE OVER 2 DECADES SINCE DREDGING AND EXCAVATION OF THE LOWER BORO-RIVER, OKAVANGO DELTA, BOTSWANA

The Okavango Delta, southern Africa's largest wetland, is situated on the fringe of the semi-arid Kalahari Desert. It is a large alluvial fa n, occupying a graben structure which is an extension of the East Afri can Rift system. Of the 16 km(3) of water which enters the Delta each year, 96% is lost to the atmosphere by evapotranspiration, 2% to groun dwater and only 2% leaves as surface flow In order to increase surface outflow to meet human needs, the distal Bore channel-floodplain syste m was dredged, excavated and bunded between 1971 and 1974. The immedia te impact of these measures was the destruction of in-channel flora. A fter 20 years, the aquatic flora has recovered in the excavated channe l. However, little recovery has occurred along the channel reach which was dredged. Moreover, there has been significant encroachment of ter restrial species onto the floodplain in the region of the dredged chan nel. In addition, dredging created a nick point which has been migrati ng upstream by headward erosion since dredging ceased. The average rat e of advance of the nick point has been about 500 m per year. Incision associated with nick point migration has produced a channel which is indistinguishable in form from the dredged channel and, like the dredg ed channel, is almost completely devoid of in-channel aquatic flora. T he adverse environmental impact of dredging has therefore continued to propagate in an upstream direction, although the height of the nick p oint has decreased, suggesting a natural attenuation process. This is likely to result in eventual elimination of the nick point.